[SR-449] swift-corelibs-libdispatch doesn't build on El Capitan

Issue Description:

$ brew install libtool automake autoconf
$./autogen.sh
$ ./configure
$ make

/bin/sh ../libtool  --tag=CC   --mode=compile clang -DHAVE_CONFIG_H -I. -I../config  -I.. -I.. -I../private -I../os  -Wall -fvisibility=hidden -momit-leaf-frame-pointer    -fblocks -g -O2 -c -o allocator.lo allocator.c
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../config -I.. -I.. -I../private -I../os -Wall -fvisibility=hidden -momit-leaf-frame-pointer -fblocks -g -O2 -c allocator.c  -fno-common -DPIC -o .libs/allocator.o
In file included from allocator.c:21:
In file included from ./internal.h:138:
../private/voucher_activity_private.h:493:10: fatal error: 'os/lock_private.h' file not found
#include <os/lock_private.h>
         ^
1 error generated.
make[2]: *** [allocator.lo] Error 1
make[1]: *** [all] Error 2
make: *** [all-recursive] Error 1

Reproduce the failure on OS X 10.11.2 using autogen.sh, configure, and make. Start with allocator.c and follow the includes through internal.h to private/voucher_activity_private.h and the missing os/lock_private.h; done means the project builds successfully on El Capitan.