Summary:
The CMakeLists.txt file finds the sources and headers for the main folly
library by finding all *.cpp and *.h files, and then excluding specific ones.

The CMakeLists.txt file was originally written for Windows builds, and it was
hard-coded to exclude a number of files that do not build on Windows.   This
updates the file to properly check for the dependencies, and build these files
if their required third-party dependencies are available.

Reviewed By: meyering

Differential Revision: D6745368

fbshipit-source-id: 3ee04d54147a68c482aab50d30bc9fe2a4134eef