Allow overriding version number when building
Allow overriding version number when building

Summary:
I want to continuously build the latest version of Watchman. I want unique version numbers, so allow CI systems to configure Watchman's version number:

  $ cmake -DWATCHMAN_VERSION_OVERRIDE="4.9.4.185"

Without `-DWATCHMAN_VERSION_OVERRIDE`, this diff should not change behaviour.

Reviewed By: wez

Differential Revision: D9587792

fbshipit-source-id: f54f2fcc456f63e5c8f8d2fd890c690ea8b7b23a

wez/watchman,facebook/watchman,wez/watchman,wez/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,wez/watchman,wez/watchman,nodakai/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,wez/watchman,wez/watchman