Add git betaversion to VS-Desktop filenames

* configure.ac: Substitute git_betaversion
* src/gpg4win.mk.in: Use VSD_VERSION_FILE for output file names.

The internal MSI version is not changed as it needs to be in
the scheme x.y.z.k
but this allows us to identify test versions in the about
dialog and in the filename without bumping the k version
every time.