constexpr: Use GLIBCXX instead of GNUC (#1288)
constexpr: Use GLIBCXX instead of GNUC (#1288)

Summary:
libcxx does not have strcmp or strlen as constexpr.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Pull Request resolved: https://github.com/facebook/folly/pull/1288

Reviewed By: Orvid

Differential Revision: D19226813

Pulled By: yfeldblum

fbshipit-source-id: 4b5de4f923f4d7871c059af54eb7d375f344f48a

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly