configure: require --without-plymouth to disable plymouth support
    
    Plymouth is used pretty widely these days, and we're getting bugs
    from people who didn't have the buildrequires, didn't get plymouth
    support and were left with a broken setup.
    
    Instead, just just let the distros that don't use plymouth opt out.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=740802