Summary:
The absence of GFlags is already partially optional -
portability/GFlags.h defines a number of stub macros / clases if
FOLLY_HAVE_LIBGFLAGS is false.

However the top-level CMake rules don't correctly handle when GFlags
isn't present - they unconditionally add GFlags-related variables to
CMAKE_REQUIRED_xxx variables.

Additionally there are a couple of source files which should not be
compiled if GFlags isn't available.
Pull Request resolved: https://github.com/facebook/folly/pull/1325

Differential Revision: D20252030

Pulled By: yfeldblum

fbshipit-source-id: 7c3441e8b2850d68df125fb9c14b8417f4d08b55