[profile] Build with -fms-extensions in MinGW mode

[mstorsjo]

Summary

54902e00d1280785042e2c7ec71b25a949ade92c added a use of
/alternatename via a #pragma comment(linker); in MinGW mode, this
requires building with -fms-extensions. (This flag is added to
SANITIZER_COMMON_CFLAGS in the toplevel CMakeLists.txt.)

This avoids a warning when building in MinGW mode (about an unknown
pragma being ignored), and presumably also makes the code work as
intended.

[phosek]

LGTM

[nikic]

From what I can tell, this only works if clang is used as the compiler. While GCC also supports -fms-extensions, it does not support #pragma comment(linker, ...). We're seeing linker errors when linking against a gcc-built profiler runtime on mingw targets.

Is it possible to use the non-_WIN32 codepath on mingw?

compiler-rt/lib/profile/CMakeLists.txt

set(EXTRA_FLAGS
      ${EXTRA_FLAGS}
      -DCOMPILER_RT_HAS_UNAME=1)
 endif()
 
 # We don't use the C++ Standard Library here, so avoid including it by mistake.
 append_list_if(COMPILER_RT_HAS_NOSTDINCXX_FLAG -nostdinc++ EXTRA_FLAGS)
 # XRay uses C++ standard library headers.
 string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+# The Windows specific code uses a #pragma comment(linker, ...) which requires
+# -fms-extensions on MinGW targets.
+append_list_if(MINGW -fms-extensions EXTRA_FLAGS)
 
 # This appears to be a C-only warning banning the use of locals in aggregate
 # initializers. All other compilers accept this, though.
 # nonstandard extension used : 'identifier' : cannot be initialized using address of automatic variable
 append_list_if(COMPILER_RT_HAS_WD4221_FLAG /wd4221 EXTRA_FLAGS)
 
 if(APPLE)
   add_compiler_rt_runtime(clang_rt.profile