On buildbots the build fails because git isn't finding any git repo and
then AC_INIT refuses to run:

fatal: not a git repository (or any parent up to mount point /)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
configure.ac:5: error: AC_INIT should be called with package and version arguments

Address this by substituting the git command with $(PKG_VERSION).

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>