As per the comments in git-version-gen, the .version file that is
typically generated automatically during build should be included
in the tarball, so generate it in the dist-hook.
This fixes a problem when running make distcheck which uses a read-only
source folder and thus cannot generate the $top_srcdir/.version as
part of it's build process (it uses a _build folder which is r/w)
and thus cannot be built.
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.
