Make VERSION generation more robust.

Relax the "are we in a git repo?" check to succeed even if the top level
jemalloc directory is not at the top level of the git repo.

Add git tag filtering so that only version triplets match when
generating VERSION.

Add fallback bogus VERSION creation, so that in the worst case, rather
than generating empty values for e.g. JEMALLOC_VERSION_MAJOR,
configuration ends up generating useless constants.