Summary:
- Since folly requires GCC 5+ compiler, remove several macros which were
  guarding against when we supported GCC 4.9.
Pull Request resolved: https://github.com/facebook/folly/pull/1096

Reviewed By: LeeHowes

Differential Revision: D14681043

Pulled By: LeeHowes

fbshipit-source-id: fcc2cd25e00c512f7154a1568003e7c2f0df9c54