cmake: use -std=gnu++14 when performing configure checks

Summary:
Always use the `-std=gnu++14` compiler flag when performing configure-time
checks on Unix-based platforms.

On older versions of gcc (e.g. 5.x) some of the configuration checks
incorrectly fail without this option.

Reviewed By: meyering

Differential Revision: D6788847

fbshipit-source-id: 6e182ff9930a42b01c23f9a0cceda0108052224b

CMake/FollyCompilerUnix.cmake

@@ -2,6 +2,7 @@ set(CMAKE_CXX_FLAGS_COMMON -g -Wall -Wextra)
 set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_COMMON}")
 set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_COMMON} -O3")
 
+list(APPEND CMAKE_REQUIRED_FLAGS -std=gnu++14)
 function(apply_folly_compile_options_to_target THETARGET)
   target_compile_options(${THETARGET}
     PUBLIC