spec: Use %license when available
    
    This is required by the fedora packaging guidelines:
    
    https://fedoraproject.org/wiki/Packaging:LicensingGuidelines
    
    This macro isn't available on stock RHEL6 so provide a backcompat
    definition
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1483293
    
    Reported-by: Ville Skyttä <ville.skytta@iki.fi>