[CMake][libcxx] Use check_c_compiler_flag to check for nodefaultlibs

[phosek]

Summary

We're using -nodefaultlibs to avoid the dependency on C++ library when using check_cxx_compiler_flag, and as such we cannot use check_cxx_compiler_flag to check the availability of -nodefaultlibs itself, because check_cxx_compile_flags adds the flag to COMPILE_DEFINITIONS when calling try_compile and this variable is only considered for compilation but not for linking where -nodefaultlibs actually makes a difference.

[EricWF]

Why not? The executable should still link and run without requiring the C++ standard library, just as it links and runs without requiring the C standard library.

[phosek]

The problem is that check_cxx_compile_flags adds the flag to COMPILE_DEFINITIONSwhen calling try_compile and that variable is only considered for compilation but not for linking where -nodefaultlibs actually makes a difference (I should probably add this into the description).

[EricWF]

Is there any CMake documentation which documents this particular behavior?

Otherwise LGTM.

[phosek]

https://cmake.org/cmake/help/latest/module/CheckCXXCompilerFlag.html says that "This internally calls the check_cxx_source_compiles macro and sets CMAKE_REQUIRED_DEFINITIONS to <flag>" and https://github.com/Kitware/CMake/blob/master/Modules/CheckCXXSourceCompiles.cmake#L67 shows that this is being passed to try_compile as COMPILE_DEFINITIONS, this is the best I could find.

cmake/config-ix.cmake

 include(CheckLibraryExists)
+include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 
 if(WIN32 AND NOT MINGW)
   # NOTE(compnerd) this is technically a lie, there is msvcrt, but for now, lets
   # let the default linking take care of that.
   set(LIBCXX_HAS_C_LIB NO)
 else()
   check_library_exists(c fopen "" LIBCXX_HAS_C_LIB)

 
 # libc++ is built with -nodefaultlibs, so we want all our checks to also
 # use this option, otherwise we may end up with an inconsistency between
 # the flags we think we require during configuration (if the checks are
 # performed without -nodefaultlibs) and the flags that are actually
 # required during compilation (which has the -nodefaultlibs). libc is
 # required for the link to go through. We remove sanitizers from the
 # configuration checks to avoid spurious link errors.
-check_cxx_compiler_flag(-nodefaultlibs LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
+check_c_compiler_flag(-nodefaultlibs LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
 if (LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
   if (LIBCXX_HAS_C_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
   if (LIBCXX_USE_COMPILER_RT)
     list(APPEND CMAKE_REQUIRED_FLAGS -rtlib=compiler-rt)
     find_compiler_rt_library(builtins LIBCXX_BUILTINS_LIBRARY)