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