Add UTC date to GIT snapshot version generation
    
    This way make rpms will always generate new packages that can be installed on
    top fo older ones, regardless of alphabetic ordering of the GIT commit id.
    Also make sure version and date variables are immditely resolved, so they can't
    change during the build.
    
    Reviewed-By: Nathaniel McCallum <npmccallum@redhat.com>