Summary:
This diff allows passing a watchman version number override
via the environment as well as via the cmake `WATCHMAN_VERSION_OVERRIDE`
option.

To help invalidate the build I've added a new section to the manifest
files that allows listing out additional env vars that the project
hashes should be sensitive to.  The effect of this is that we'll
re-run the cmake configure step if the listed env vars are changed.

Reviewed By: Ben0mega

Differential Revision: D17865896

fbshipit-source-id: 8ea5572b0b9b7af95ec5c310e494cb17a139ced4