cmake: Move version setup into a separate .cmake file

cmake: Move version setup into a separate .cmake file

Introduce a new QuasselVersion.cmake that contains the hard-coded
version, as well as the code for retrieving version information from
git (if available). This file also sets up version.h with the given
information. Remove the corresponding code from the main
CMakeLists.txt.

Output the git revision at configure time if available, which helps
when analyzing build logs.