[crt] Use -std=c11 for crtbegin.o/crtend.o

[phosek]

Summary

The source uses C11 syntax such as comments and some compilers print
warnings without specifying this flag.

[thakis]

lgtm, but shouldn't we set this globally for all projects?

[phosek]

In D61797#1498367, @thakis wrote:

lgtm, but shouldn't we set this globally for all projects?

I've noticed that builtins use C11 (https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/builtins/CMakeLists.txt#L575) so I've done the same here. I'm fine unifying this but I'd prefer to do it in a followup.

compiler-rt/trunk/cmake/config-ix.cmake

set(MINGW_LIBRARIES mingw32 ${MINGW_RUNTIME} moldname mingwex msvcrt advapi32
                         shell32 user32 kernel32 mingw32 ${MINGW_RUNTIME}
                         moldname mingwex msvcrt)
     list(APPEND CMAKE_REQUIRED_LIBRARIES ${MINGW_LIBRARIES})
   endif()
 endif ()
 
 # CodeGen options.
 check_c_compiler_flag(-ffreestanding         COMPILER_RT_HAS_FFREESTANDING_FLAG)
+check_c_compiler_flag(-std=c11               COMPILER_RT_HAS_STD_C11_FLAG)
 check_cxx_compiler_flag(-fPIC                COMPILER_RT_HAS_FPIC_FLAG)
 check_cxx_compiler_flag(-fPIE                COMPILER_RT_HAS_FPIE_FLAG)
 check_cxx_compiler_flag(-fno-builtin         COMPILER_RT_HAS_FNO_BUILTIN_FLAG)
 check_cxx_compiler_flag(-fno-exceptions      COMPILER_RT_HAS_FNO_EXCEPTIONS_FLAG)
 check_cxx_compiler_flag(-fomit-frame-pointer COMPILER_RT_HAS_FOMIT_FRAME_POINTER_FLAG)
 check_cxx_compiler_flag(-funwind-tables      COMPILER_RT_HAS_FUNWIND_TABLES_FLAG)
 check_cxx_compiler_flag(-fno-stack-protector COMPILER_RT_HAS_FNO_STACK_PROTECTOR_FLAG)
 check_cxx_compiler_flag(-fno-sanitize=safe-stack COMPILER_RT_HAS_FNO_SANITIZE_SAFE_STACK_FLAG)

compiler-rt/trunk/lib/crt/CMakeLists.txt

function(check_cxx_section_exists section output)
   endif()
 
   file(REMOVE_RECURSE ${TARGET_NAME})
 endfunction()
 
 check_cxx_section_exists(".init_array" COMPILER_RT_HAS_INITFINI_ARRAY
   SOURCE "__attribute__((constructor)) void f() {}\nint main() { return 0; }\n")
 
+append_list_if(COMPILER_RT_HAS_STD_C11_FLAG -std=c11 CRT_CFLAGS)
 append_list_if(COMPILER_RT_HAS_INITFINI_ARRAY -DCRT_HAS_INITFINI_ARRAY CRT_CFLAGS)
 append_list_if(COMPILER_RT_HAS_FPIC_FLAG -fPIC CRT_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WNO_PEDANTIC -Wno-pedantic CRT_CFLAGS)
 
 foreach(arch ${CRT_SUPPORTED_ARCH})
   add_compiler_rt_runtime(clang_rt.crtbegin
     OBJECT
     ARCHS ${arch}