really require the exact jbig2dec version
    
    673c823 ("Require the exact jbig2dec version in both build and runtime
    dependencies", 2020-06-24) introduced a lower bound for the version. But
    apparantly ghostscript complains even about newer versions (see e.g.
    https://bodhi.fedoraproject.org/updates/FEDORA-2020-df4550580d).
    
    So, make this a check on the exact jbig2dec version (but not the full
    package version).