Summary:
Use the watchman cmake files to seed the common cmake library and adjust
a number of our opensource projects to take their glog and gflags module
finding from that central location.

Note that there are a variety of ad-hoc re-implementations of shipit in various build scripts used in sandcastle jobs at the moment that result in adding multiple cmake module paths to correctly locate the shared cmake directory.  The long term solution is to get all those projects integrated in the new fbcode_builder stuff that I'm working on.

Reviewed By: simpkins, calebmarchent

Differential Revision: D14549654

fbshipit-source-id: e49a7f20a15af52b4b3ed3037b8973fc293656dc