Summary:
Some changes that had to be done in Debian for the build to work:

* Installation of system deps failed as it was not detecting the OS correctly
* Updated the CMake manifest to uncomment the deb dependency as the CMake versions are newer
* fb303 was not getting cloned as it was looking for a non-existent master branch

Pull Request resolved: https://github.com/facebook/watchman/pull/943

Differential Revision: D30700180

Pulled By: fanzeyi

fbshipit-source-id: 62eb9f8ba57510eb98d6bd56e40384bf38a74d8c