build-sys: Add some smarts to version extraction from git tags.
When we are dealing with a tag against a commit with other tags applied,
then favour the first tag, not the last one. This allows us to tag a release
as e.g. v1.0, v1.1-dev and v2.0-dev and get a version of 1.0 rather than the
newer tags.
This also removes support for git versions that do not accept
the --match argument as we also now make use of the --contains arg
which was added at the same time (I think)
Additionally, the v variable is reset just incase it is already exported
in the shell for some reason.