[SR-5969] Swift build error on Gentoo

Issue Description:

The log is:

– The ASM compiler identification is unknown
– Found assembler: /usr/bin/clang
– Warning: Did not find file Compiler/ASM
- Finding installed version for: cmake
– Found version: cmake version 3.7.2 CMake suite maintained and supported by Kitware (kitware.com/cmake).
–
– Finding version for: /usr/bin/cmake
– Found version: 3.7.2
–
– Finding installed version for: ninja
– Found version: 1.7.2
–
– Finding version for: /mnt/swift/swift-source/build/buildbot_linux/ninja-build/ninja
– Found version: 1.8.2
–
– Finding version for: /usr/bin/clang
– Found version: 3.9.1
–
– Finding version for: /usr/bin/clang++
– Found version: 3.9.1
–
– Linker detection: GNU ld
– Building with fPIC
- Linux SDK:
– Path: /
– Version: don't use
– Build number: don't use
– Deployment version: don't use
– Library subdir: linux
– Version min name:
– Triple name: linux
– Architectures: x86_64
– Object Format: ELF
– Linker (x86_64): /usr/bin/ld
– Triple for x86_64 is x86_64-unknown-linux-gnu
–
– Building host Swift tools for LINUX x86_64
– Build type: Release
– Assertions: TRUE
– LTO:
–
– Building Swift standard library and SDK overlays for SDKs: LINUX
– Build type: Release
– Assertions: FALSE
–
– Building Swift runtime with:
– Leak Detection Checker Entrypoints: FALSE
–
– Found ICU: /usr/include
– Swift version: 4.1
– Swift vendor:
– Swift compiler version:
– Embedded clang compiler version:
– Found ICU: /usr/include
CMake Warning at docs/CMakeLists.txt:130 (message):
LitRe not found; code examples won't be tested.

– Configuring done
– Generating done
– Build files have been written to: /mnt/swift/swift-source/build/buildbot_linux/swift-linux-x86_64

• popd
  /mnt/swift/swift-source/swift
• /usr/bin/cmake --build /mnt/swift/swift-source/build/buildbot_linux/swift-linux-x86_64 – -j4 all swift-stdlib-linux-x86_64
  [2/230] Performing build step for 'libdispatch'
  FAILED: tools/SourceKit/libdispatch-prefix/src/libdispatch-stamp/libdispatch-build /mnt/swift/swift-source/build/buildbot_linux/libdispatch-linux-x86_64/src/libdispatch.so
  cd /mnt/swift/swift-source/build/buildbot_linux/libdispatch-linux-x86_64 && /usr/bin/cmake --build . && /usr/bin/cmake -E touch /mnt/swift/swift-source/build/buildbot_linux/swift-linux-x86_64/tools/SourceKit/libdispatch-prefix/src/libdispatch-stamp/libdispatch-build
  [1/51] cd /mnt/swift/swift-source/build/buildbot_linux/libdispatch-linux-x86_64 && /usr/bin/cmake -E create_symlink /mnt/swift/swift-source/swift-corelibs-libdispatch/dispatch/generic/module.modulemap /mnt/swift/swift-source/swift-corelibs-libdispatch/dispatch/module.modulemap && /usr/bin/cmake -E create_symlink /mnt/swift/swift-source/swift-corelibs-libdispatch/private/generic/module.modulemap /mnt/swift/swift-source/swift-corelibs-libdispatch/private/module.modulemap
  [2/51] Building C object tests/CMakeFiles/bsdtests.dir/dispatch_test.c.o
  FAILED: tests/CMakeFiles/bsdtests.dir/dispatch_test.c.o
  /mnt/swift/swift-source/build/buildbot_linux/llvm-linux-x86_64/bin/clang -DDISPATCH_USE_DTRACE=1 -DHAVE_CONFIG_H -Itests -I/mnt/swift/swift-source/swift-corelibs-libdispatch/tests -I/mnt/swift/swift-source/swift-corelibs-libdispatch -march=native -pipe -O2 -fomit-frame-pointer -fvisibility=hidden -Werror -Wall -Wextra -Wno-unknown-warning-option -Wno-trigraphs -Wmissing-field-initializers -Wmissing-prototypes -Wdocumentation -Wunreachable-code -Wmissing-braces -Wparentheses -Wswitch -Wunused-function -Wunused-label -Wunused-parameter -Wunused-variable -Wunused-value -Wempty-body -Wuninitialized -Wunknown-pragmas -Wshadow -Wno-four-char-constants -Wconversion -Wconstant-conversion -Wint-conversion -Wbool-conversion -Wenum-conversion -Wassign-enum -Wsign-compare -Wshorten-64-to-32 -Wpointer-sign -Wnewline-eof -Wdeprecated-declarations -Wsign-conversion -Winfinite-recursion -Warray-bounds-pointer-arithmetic -Watomic-properties -Wcomma -Wconditional-uninitialized -Wcovered-switch-default -Wdate-time -Wdeprecated -Wdouble-promotion -Wduplicate-enum -Wexpansion-to-defined -Wfloat-equal -Widiomatic-parentheses -Wignored-qualifiers -Wimplicit-fallthrough -Wnullable-to-nonnull-conversion -Wobjc-interface-ivars -Wover-aligned -Wpacked -Wpointer-arith -Wselector -Wstatic-in-inline -Wsuper-class-method-mismatch -Wswitch-enum -Wtautological-compare -Wunguarded-availability -Wunused -Wno-disabled-macro-expansion -Wno-pedantic -Wno-bad-function-cast -Wno-c+-compat -Wno-c 98-compat -Wno-c+98-compat-pedantic -Wno-cast-align -Wno-cast-qual -Wno-documentation-unknown-command -Wno-format-nonliteral -Wno-missing-variable-declarations -Wno-old-style-cast -Wno-padded -Wno-reserved-id-macro -Wno-shift-sign-overflow -Wno-undef -Wno-unreachable-code-aggressive -Wno-unused-macros -Wno-used-but-marked-unused -Wno-vla -DLIBBSD_OVERLAY -isystem /usr/local/include/bsd -MD -MT tests/CMakeFiles/bsdtests.dir/dispatch_test.c.o -MF tests/CMakeFiles/bsdtests.dir/dispatch_test.c.o.d -o tests/CMakeFiles/bsdtests.dir/dispatch_test.c.o -c /mnt/swift/swift-source/swift-corelibs-libdispatch/tests/dispatch_test.c
  In file included from /mnt/swift/swift-source/swift-corelibs-libdispatch/tests/dispatch_test.c:22:
  /mnt/swift/swift-source/swift-corelibs-libdispatch/tests/bsdtests.h:72:72: error: expected function body after function declarator
  void test_ptr_notnull_format(const void *ptr, const char *format, ...) __printflike(2, 3);
  ^
  /mnt/swift/swift-source/swift-corelibs-libdispatch/tests/bsdtests.h:80:89: error: expected function body after function declarator
  void test_ptr_format(const void* actual, const void* expected, const char *format, ...) __printflike(3,4);
  ^
  /mnt/swift/swift-source/swift-corelibs-libdispatch/tests/bsdtests.h:84:86: error: expected function body after function declarator
  void test_uint32_format(uint32_t actual, uint32_t expected, const char *format, ...) __printflike(3,4);
  ^
  /mnt/swift/swift-source/swift-corelibs-libdispatch/tests/bsdtests.h:88:83: error: expected function body after function declarator
  void test_int32_format(int32_t actual, int32_t expected, const char* format, ...) __printflike(3,4);

....

Start with swift-corelibs-libdispatch/tests/bsdtests.h and the failing tests/dispatch_test.c compilation shown in the log. Reproduce the libdispatch build with the provided Gentoo environment and inspect the compiler invocation around the __printflike declarations. Done means the libdispatch build and its tests compile successfully on the reported Linux setup.