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.
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