Add kGnuc C++ constant representing __GNUC__
Add kGnuc C++ constant representing __GNUC__

Summary: [Folly] Add `kGnuc` C++ constant representing `__GNUC__` preprocessor constant.

Reviewed By: vitaut

Differential Revision: D18827910

fbshipit-source-id: 43e07e12017df1e7cf4d6a5747e7a85d8a7616f3

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