Rather than relying on always having a full cloned repository when
building, add fallbacks which work for trees with constrained (such
as shallow-cloned trees), or no, history. When the tagged commit is
reachable, use git describe, else rely on the version string in
configure since that's required to always be there regardless of
distribution mode. If we have git tree then include the HEAD sha1
as well.

While there, clean up versioning to more clearly separate us from
upstream.