[CMake] Use -std=c++11 if supported

[beanz]

Summary

This patch adds a check for if -std=c++11 is a supported flag, and adds it to CMAKE_CXX_FLAGS if it is supported.

[EricWF]

LGTM. I'm surprised we don't already require C++11.

cmake/config-ix.cmake

40 ↗

(On Diff #69619)

Nit LIBCXXABI_HAS_STD_CXX11.

libcxxabi/trunk/cmake/config-ix.cmake

 check_cxx_compiler_flag(-Werror               LIBCXXABI_HAS_WERROR_FLAG)
 check_cxx_compiler_flag(-Wno-error            LIBCXXABI_HAS_WNO_ERROR_FLAG)
 check_cxx_compiler_flag(/WX                   LIBCXXABI_HAS_WX_FLAG)
 check_cxx_compiler_flag(/WX-                  LIBCXXABI_HAS_NO_WX_FLAG)
 check_cxx_compiler_flag(/EHsc                 LIBCXXABI_HAS_EHSC_FLAG)
 check_cxx_compiler_flag(/EHs-                 LIBCXXABI_HAS_NO_EHS_FLAG)
 check_cxx_compiler_flag(/EHa-                 LIBCXXABI_HAS_NO_EHA_FLAG)
 check_cxx_compiler_flag(/GR-                  LIBCXXABI_HAS_NO_GR_FLAG)
+check_cxx_compiler_flag(-std=c++11            LIBCXXABI_HAS_STD_CXX11)
+
+if(LIBCXXABI_HAS_STD_CXX11)
+  list(APPEND CMAKE_CXX_FLAGS -std=c++11)
+endif()
 
 # Check libraries
 check_library_exists(c fopen "" LIBCXXABI_HAS_C_LIB)
 check_library_exists(dl dladdr "" LIBCXXABI_HAS_DL_LIB)
 check_library_exists(pthread pthread_once "" LIBCXXABI_HAS_PTHREAD_LIB)
 if (NOT LIBCXXABI_USE_COMPILER_RT)
   check_library_exists(gcc_s __gcc_personality_v0 "" LIBCXXABI_HAS_GCC_S_LIB)
 endif ()
 check_library_exists(c __cxa_thread_atexit_impl ""
   LIBCXXABI_HAS_CXA_THREAD_ATEXIT_IMPL)