[build] Use git-version-gen during specfile build

Instead of copying parts of git-version-gen for spec target use
git-version-gen directly and parse final version into components
(rpmver, alphatag, numcomm) and use them.

Main reason is to simplify code a bit (sed scripts are a bit repetitive
tho), reuse the code and also allow building of RPM from dist tarball
generated from non-tagged commit or dirty git (not very useful).

The code relies on fact, that hyphen is never used in tagged release
name.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>