[SR-4505] Minor warning in libdispatch build

Issue Description:

I see ar: `u' modifier ignored since `D' is the default (see `U') errors in the build. Nothing significant, just thought someone might want to clean it up a bit. Ubuntu 16.04.

rmann@rlmubuntu: swift-corelibs-libdispatch/ (master) 12:14$ make
Making all in dispatch
make[1]: Entering directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/dispatch'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/dispatch'
Making all in libpwq
make[1]: Entering directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/libpwq'
make  all-am
make[2]: Entering directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/libpwq'
/bin/bash ./libtool  --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I.    -I./include -I./src -Wall -Wextra -Werror -D_XOPEN_SOURCE=600 -D__EXTENSIONS__ -D_GNU_SOURCE -std=c99 -fvisibility=hidden -g -O2 -MT src/linux/libpthread_workqueue_la-thread_info.lo -MD -MP -MF src/linux/.deps/libpthread_workqueue_la-thread_info.Tpo -c -o src/linux/libpthread_workqueue_la-thread_info.lo `test -f 'src/linux/thread_info.c' || echo './'`src/linux/thread_info.c
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I./include -I./src -Wall -Wextra -Werror -D_XOPEN_SOURCE=600 -D__EXTENSIONS__ -D_GNU_SOURCE -std=c99 -fvisibility=hidden -g -O2 -MT src/linux/libpthread_workqueue_la-thread_info.lo -MD -MP -MF src/linux/.deps/libpthread_workqueue_la-thread_info.Tpo -c src/linux/thread_info.c  -fPIC -DPIC -o src/linux/.libs/libpthread_workqueue_la-thread_info.o
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I./include -I./src -Wall -Wextra -Werror -D_XOPEN_SOURCE=600 -D__EXTENSIONS__ -D_GNU_SOURCE -std=c99 -fvisibility=hidden -g -O2 -MT src/linux/libpthread_workqueue_la-thread_info.lo -MD -MP -MF src/linux/.deps/libpthread_workqueue_la-thread_info.Tpo -c src/linux/thread_info.c -o src/linux/libpthread_workqueue_la-thread_info.o >/dev/null 2>&1
mv -f src/linux/.deps/libpthread_workqueue_la-thread_info.Tpo src/linux/.deps/libpthread_workqueue_la-thread_info.Plo
/bin/bash ./libtool  --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I.    -I./include -I./src -Wall -Wextra -Werror -D_XOPEN_SOURCE=600 -D__EXTENSIONS__ -D_GNU_SOURCE -std=c99 -fvisibility=hidden -g -O2 -MT src/linux/libpthread_workqueue_la-thread_rt.lo -MD -MP -MF src/linux/.deps/libpthread_workqueue_la-thread_rt.Tpo -c -o src/linux/libpthread_workqueue_la-thread_rt.lo `test -f 'src/linux/thread_rt.c' || echo './'`src/linux/thread_rt.c
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I./include -I./src -Wall -Wextra -Werror -D_XOPEN_SOURCE=600 -D__EXTENSIONS__ -D_GNU_SOURCE -std=c99 -fvisibility=hidden -g -O2 -MT src/linux/libpthread_workqueue_la-thread_rt.lo -MD -MP -MF src/linux/.deps/libpthread_workqueue_la-thread_rt.Tpo -c src/linux/thread_rt.c  -fPIC -DPIC -o src/linux/.libs/libpthread_workqueue_la-thread_rt.o
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I./include -I./src -Wall -Wextra -Werror -D_XOPEN_SOURCE=600 -D__EXTENSIONS__ -D_GNU_SOURCE -std=c99 -fvisibility=hidden -g -O2 -MT src/linux/libpthread_workqueue_la-thread_rt.lo -MD -MP -MF src/linux/.deps/libpthread_workqueue_la-thread_rt.Tpo -c src/linux/thread_rt.c -o src/linux/libpthread_workqueue_la-thread_rt.o >/dev/null 2>&1
mv -f src/linux/.deps/libpthread_workqueue_la-thread_rt.Tpo src/linux/.deps/libpthread_workqueue_la-thread_rt.Plo
/bin/bash ./libtool  --tag=CC   --mode=link gcc -I./include -I./src -Wall -Wextra -Werror -D_XOPEN_SOURCE=600 -D__EXTENSIONS__ -D_GNU_SOURCE -std=c99 -fvisibility=hidden -g -O2   -o libpthread_workqueue.la  src/libpthread_workqueue_la-api.lo src/libpthread_workqueue_la-witem_cache.lo src/posix/libpthread_workqueue_la-manager.lo src/posix/libpthread_workqueue_la-thread_info.lo src/posix/libpthread_workqueue_la-thread_rt.lo src/linux/libpthread_workqueue_la-load.lo src/linux/libpthread_workqueue_la-thread_info.lo src/linux/libpthread_workqueue_la-thread_rt.lo -lpthread -lrt 
copying selected object files to avoid basename conflicts...
libtool: link: ln src/linux/.libs/libpthread_workqueue_la-thread_info.o .libs/libpthread_workqueue.lax/lt1-libpthread_workqueue_la-thread_info.o || cp src/linux/.libs/libpthread_workqueue_la-thread_info.o .libs/libpthread_workqueue.lax/lt1-libpthread_workqueue_la-thread_info.o
libtool: link: ln src/linux/.libs/libpthread_workqueue_la-thread_rt.o .libs/libpthread_workqueue.lax/lt2-libpthread_workqueue_la-thread_rt.o || cp src/linux/.libs/libpthread_workqueue_la-thread_rt.o .libs/libpthread_workqueue.lax/lt2-libpthread_workqueue_la-thread_rt.o
libtool: link: ar cru .libs/libpthread_workqueue.a src/.libs/libpthread_workqueue_la-api.o src/.libs/libpthread_workqueue_la-witem_cache.o src/posix/.libs/libpthread_workqueue_la-manager.o src/posix/.libs/libpthread_workqueue_la-thread_info.o src/posix/.libs/libpthread_workqueue_la-thread_rt.o src/linux/.libs/libpthread_workqueue_la-load.o .libs/libpthread_workqueue.lax/lt1-libpthread_workqueue_la-thread_info.o .libs/libpthread_workqueue.lax/lt2-libpthread_workqueue_la-thread_rt.o
ar: `u' modifier ignored since `D' is the default (see `U')
libtool: link: ranlib .libs/libpthread_workqueue.a
libtool: link: rm -fr .libs/libpthread_workqueue.lax
libtool: link: ( cd ".libs" && rm -f "libpthread_workqueue.la" && ln -s "../libpthread_workqueue.la" "libpthread_workqueue.la" )
make[2]: Leaving directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/libpwq'
make[1]: Leaving directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/libpwq'
Making all in man
make[1]: Entering directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/man'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/man'
Making all in os
make[1]: Entering directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/os'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/os'
Making all in private
make[1]: Entering directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/private'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/private'
Making all in src
make[1]: Entering directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/src'
/usr/bin/dtrace -h -s /home/rmann/libdispatch/swift-corelibs-libdispatch/src/provider.d -o provider.h
make  all-am
make[2]: Entering directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/src'
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o allocator.lo allocator.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c allocator.c  -fPIC -DPIC -o .libs/allocator.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o apply.lo apply.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c apply.c  -fPIC -DPIC -o .libs/apply.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o benchmark.lo benchmark.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c benchmark.c  -fPIC -DPIC -o .libs/benchmark.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o data.lo data.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c data.c  -fPIC -DPIC -o .libs/data.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o init.lo init.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c init.c  -fPIC -DPIC -o .libs/init.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o introspection.lo introspection.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c introspection.c  -fPIC -DPIC -o .libs/introspection.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o io.lo io.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c io.c  -fPIC -DPIC -o .libs/io.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o mach.lo mach.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c mach.c  -fPIC -DPIC -o .libs/mach.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o object.lo object.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c object.c  -fPIC -DPIC -o .libs/object.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o once.lo once.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c once.c  -fPIC -DPIC -o .libs/once.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o queue.lo queue.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c queue.c  -fPIC -DPIC -o .libs/queue.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o semaphore.lo semaphore.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c semaphore.c  -fPIC -DPIC -o .libs/semaphore.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o source.lo source.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c source.c  -fPIC -DPIC -o .libs/source.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o time.lo time.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c time.c  -fPIC -DPIC -o .libs/time.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o transform.lo transform.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c transform.c  -fPIC -DPIC -o .libs/transform.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o voucher.lo voucher.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c voucher.c  -fPIC -DPIC -o .libs/voucher.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o event/event.lo event/event.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c event/event.c  -fPIC -DPIC -o event/.libs/event.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o event/event_epoll.lo event/event_epoll.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c event/event_epoll.c  -fPIC -DPIC -o event/.libs/event_epoll.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o event/event_kevent.lo event/event_kevent.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c event/event_kevent.c  -fPIC -DPIC -o event/.libs/event_kevent.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o shims/linux_stubs.lo shims/linux_stubs.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c shims/linux_stubs.c  -fPIC -DPIC -o shims/.libs/linux_stubs.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o shims/lock.lo shims/lock.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c shims/lock.c  -fPIC -DPIC -o shims/.libs/lock.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o BlocksRuntime/data.lo BlocksRuntime/data.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c BlocksRuntime/data.c  -fPIC -DPIC -o BlocksRuntime/.libs/data.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -O2 -c -o BlocksRuntime/runtime.lo BlocksRuntime/runtime.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -O2 -c BlocksRuntime/runtime.c  -fPIC -DPIC -o BlocksRuntime/.libs/runtime.o
/bin/bash ../libtool  --tag=CXX   --mode=compile clang++ -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private  -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -std=gnu++11 -fno-exceptions -O2 -c -o libdispatch_la-block.lo `test -f 'block.cpp' || echo './'`block.cpp
libtool: compile:  clang++ -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer -isystem /usr/include/bsd -DLIBBSD_OVERLAY -fblocks -I../src/BlocksRuntime -std=gnu++11 -fno-exceptions -O2 -c block.cpp  -fPIC -DPIC -o .libs/libdispatch_la-block.o
/bin/bash ../libtool  --tag=CXX   --mode=link clang++ -I../libpwq/include -Wall -fvisibility=hidden -momit-leaf-frame-pointer  -isystem /usr/include/bsd -DLIBBSD_OVERLAY  -fblocks -I../src/BlocksRuntime -std=gnu++11 -fno-exceptions -O2 -avoid-version  -Xcompiler -fuse-ld=gold   -o libdispatch.la -rpath /usr/local/lib allocator.lo apply.lo benchmark.lo data.lo init.lo introspection.lo io.lo mach.lo object.lo once.lo queue.lo semaphore.lo source.lo time.lo transform.lo voucher.lo event/event.lo event/event_epoll.lo event/event_kevent.lo shims/linux_stubs.lo shims/lock.lo BlocksRuntime/data.lo BlocksRuntime/runtime.lo  libdispatch_la-block.lo    ../libpwq/libpthread_workqueue.la -lbsd   -lpthread 
libtool: link: clang++  -fPIC -DPIC -shared -nostdlib /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../x86_64-linux-gnu/crti.o /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/crtbeginS.o  .libs/allocator.o .libs/apply.o .libs/benchmark.o .libs/data.o .libs/init.o .libs/introspection.o .libs/io.o .libs/mach.o .libs/object.o .libs/once.o .libs/queue.o .libs/semaphore.o .libs/source.o .libs/time.o .libs/transform.o .libs/voucher.o event/.libs/event.o event/.libs/event_epoll.o event/.libs/event_kevent.o shims/.libs/linux_stubs.o shims/.libs/lock.o BlocksRuntime/.libs/data.o BlocksRuntime/.libs/runtime.o .libs/libdispatch_la-block.o  -Wl,--whole-archive ../libpwq/.libs/libpthread_workqueue.a -Wl,--no-whole-archive  -lrt -lbsd -lpthread -L/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0 -L/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../x86_64-linux-gnu -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../.. -L/usr/lib/llvm-3.8/bin/../lib -L/lib -L/usr/lib -lstdc++ -lm -lc -lgcc_s /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/crtendS.o /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../x86_64-linux-gnu/crtn.o  -momit-leaf-frame-pointer -O2 -fuse-ld=gold   -Wl,-soname -Wl,libdispatch.so -o .libs/libdispatch.so
libtool: link: ( cd ".libs" && rm -f "libdispatch.la" && ln -s "../libdispatch.la" "libdispatch.la" )
make[2]: Leaving directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/src'
make[1]: Leaving directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/src'
Making all in tests
make[1]: Entering directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/tests'
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I..  -Wall -Wno-deprecated-declarations  -fblocks -I../src/BlocksRuntime -isystem /usr/include/bsd -DLIBBSD_OVERLAY   -O2 -c -o bsdtests.lo bsdtests.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -Wall -Wno-deprecated-declarations -fblocks -I../src/BlocksRuntime -isystem /usr/include/bsd -DLIBBSD_OVERLAY -O2 -c bsdtests.c  -fPIC -DPIC -o .libs/bsdtests.o
/bin/bash ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I..  -Wall -Wno-deprecated-declarations  -fblocks -I../src/BlocksRuntime -isystem /usr/include/bsd -DLIBBSD_OVERLAY   -O2 -c -o dispatch_test.lo dispatch_test.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -Wall -Wno-deprecated-declarations -fblocks -I../src/BlocksRuntime -isystem /usr/include/bsd -DLIBBSD_OVERLAY -O2 -c dispatch_test.c  -fPIC -DPIC -o .libs/dispatch_test.o
/bin/bash ../libtool  --tag=CC   --mode=link clang -Wall -Wno-deprecated-declarations  -fblocks -I../src/BlocksRuntime -isystem /usr/include/bsd -DLIBBSD_OVERLAY   -O2 -avoid-version  -o libbsdtests.la  bsdtests.lo dispatch_test.lo  -lpthread 
libtool: link: ar cru .libs/libbsdtests.a .libs/bsdtests.o .libs/dispatch_test.o 
ar: `u' modifier ignored since `D' is the default (see `U')
libtool: link: ranlib .libs/libbsdtests.a
libtool: link: ( cd ".libs" && rm -f "libbsdtests.la" && ln -s "../libbsdtests.la" "libbsdtests.la" )
make[1]: Leaving directory '/home/rmann/libdispatch/swift-corelibs-libdispatch/tests'
make[1]: Entering directory '/home/rmann/libdispatch/swift-corelibs-libdispatch'
make[1]: Nothing to be done for 'all-am'.
make[1]: Leaving directory '/home/rmann/libdispatch/swift-corelibs-libdispatch'

Reproduce the Linux build by running make in libpwq and inspect the libtool link step that invokes ar cru. Trace that command to the relevant build input, then verify that a clean build no longer emits the reported ar warning.