BUILD: Compile in git version information if available

Add dynamic version information read from the git repository.
The new const char* variables XOREOS_VERSION, XOREOS_NAMEVERSION
and XOREOS_NAMEVERSIONFULL will contain those new info.

If no git or git repository is available on build time, the default
"unrevisioned" version numbers are used.
MAIN: Add a --version command line flag