Use more libstdc++ specific macro
Use more libstdc++ specific macro

Summary:
- The __GNUC__ macro is used by other compilers like Clang and
  ICC, but ext/stdio_filebuf.h is a libstdc++ only extension.

fixes #636
Closes https://github.com/facebook/folly/pull/637

Reviewed By: yfeldblum

Differential Revision: D5434532

Pulled By: Orvid

fbshipit-source-id: ce4f966a74aa9f82ae907c67807ac87ec1588173

Orvid/folly,Orvid/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,Orvid/folly,rklabs/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,Orvid/folly,facebook/folly,Orvid/folly