Configure version.cpp at build time if git repo

[kazarmy]

As per #610 (comment), this PR makes the version.cpp.in transformation happen at build time rather than configure time if the Vampire source folder is from git (i.e. there is a proper .git subdirectory), thus ensuring that the git version number in vampire ‑‑version is always updated even if one forgets to do cmake .. before make.

I think I have tested this comprehensively, but there might be a sequence of:

1. moving the git HEAD commit; and/or
2. cmake ..; and/or
3. make

that results in unexpected behavior. Please inform me if that's the case!

References:

1. https://cmake.org/pipermail/cmake/2012-May/050227.html for the cmake -P solution.
2. https://stackoverflow.com/a/43206544 for the add_custom_target solution.

[MichaelRawson]

Nice. Thank you for figuring this out and documenting your findings, this is a solid solution.