build: Generate version numbers for git

Update the version.sh script for git so it'll generate version numbers
more useful than "UNKNOWN". At the moment it only generates the short
SHA1 name, but adding a tag as a starting point should allow more
useful output. Rather than update the Makefile logic that tried to
guess whether the svn revision had changed since the last version.h
update, just run the version.sh script from configure so the version
is updated at least at that time.