[CMake] Use CMake's find_package module to detect git

[diizzyy]

Instead of enforcing execution of git if available use CMake's find_package module to make it optional.

[diizzyy]

Counterpart to what's being available using Autotools, https://github.com/xiph/flac/blob/master/configure.ac#L369

[dg0yt]

IMO this should be complemented by replacing the use of literal git wiht "${GIT_EXECUTABLE}". User may seed this cache variable without even having git in PATH.