Add rc-release target
    
    Use these make targets for creating scratch rc builds that can be used
    before an official release. rc-release will run scratch-bumpver to bump
    the version and add a rc release with the current date and time to the
    specfile.  scratch is then run to create the tar.gz file and then mock
    is used to build a rc rpm package.
    
    No changes are committed to git, no zanata account is required and
    nothing is pushed to zanata.
    
    This also Modifes scratch-bumpver to now include the rc release number.
    
    This requires that mock is installed and the current user is a member of
    the mock group.
    
    Build the rc like this:
    git clean -xdf
    git reset --hard
    ./autogen.sh
    ./configure --disable-static --enable-introspection --enable-gtk-doc
    make rc-release
    
    If everything goes well there will be a pile of rpms in the current
    directory.