revise gflags portability
revise gflags portability

Summary: Instead of defining names in the upstream namespace when they are missing, always define names in the folly namespace.

Reviewed By: mzlee

Differential Revision: D38823515

fbshipit-source-id: 9355be0aaf45489f7a72affa01ba6ea7b7df1710

facebook/wangle,facebook/wangle,facebook/wangle