Allow building glog with clang

CMakeLists.txt

@@ -1235,9 +1235,7 @@ if(NOT GLOG_FOUND OR ${LOCAL_GFLAGS})
         set(GFLAGS_LIBRARIES_TO_LINK ${GFlags_LIBS})
         set(GFLAGS_LIBRARY_DIR_TO_LINK ${GFlags_LIBRARY_DIRS})
     endif()
-    include(CheckCXXCompilerFlag)
-    check_cxx_compiler_flag(-stdlib=libc++ HAVE_FLAG_CXX)
-    if(${HAVE_FLAG_CXX})
+    if(OS_IS_MACOSX)
         set(GFLAGS_LIBRARIES_TO_LINK "${GFLAGS_LIBRARIES_TO_LINK} -lc++")
         set(GLOG_EXPORT_CXX_LIBRARIES "export CXXFLAGS=\"-stdlib=libc++\"")
     endif()