Support static gflags library
Support static gflags library

Summary: Closes https://github.com/facebook/folly/pull/587

Reviewed By: yfeldblum

Differential Revision: D5022071

Pulled By: Orvid

fbshipit-source-id: 2506087de76ba4544bf68a330bc2a18977c65f28

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