When building from a release tag (e.g. v5.4.1), git describe was finding
the nearest ancestor tag (v5.3.8) instead of the exact tag, resulting in
version strings like 'v5.3.8-9-gdfb4b22' instead of 'v5.4.1'.
Now genbuild.sh tries --exact-match first, falling back to distance-based
describe only when not on a tagged commit.
Krystie