Query git for the repository revision and use that for the version string.

Rather than compiling in the fixed version number from the configure
file, it's more helpful for testing and debugging to directly embed
the repository's revision. We use 'git describe --tags' for this, since
for tagged revisions it just returns the tag name, and in other cases
is more readable than a raw commit id.

If git isn't present or can't find a repository to query, we fall back
to the static version number in the configure script.