Simplify version numbering using git tags

As we're determined to use git for providing version numbers we might as
well do it the right way. Take all of the version information from git
describe. This assumes that at least one annotated tag exists in the
branch relating to a release. The tag will be created using

$ git tag -a -m "Version 2.18" 2.18

so git describe will give us versions like 2.18-123-gf00baa which has
better ordering properties than the current 2.18-gf00baa scheme, which
is important for package upgrades. When HEAD matches the tag exactly, it
will just give 2.18