Fix man file pattern when building with custom _prefix
    
    Man pages are usually compressed and gets the .gz extension when
    rpmbuild invokes /usr/lib/rpm/brp-compress . But even though %_mandir is
    %{_datadir}/man and %_datadir is %{_prefix}/share , brp-compress isn't
    informed about the actual _prefix and will always only process /usr .
    That seems like a bug. For now, we will work around it.
    
    Building with custom _prefix seems to be essentially what flatpak is
    doing, but I don't know if that is an actual problem there. Being able
    to build with a custom _prefix seems like a fine goal anyway. It is not
    a problem to relax the globs.
    
    This is also documented on
    https://docs.fedoraproject.org/en-US/flatpak/troubleshooting/#_uncompressed_manual_pages .