Log Message:
Updated ghostscript-agpl to 9.20.

Remove Makefile.common that is not used by anything, despite
a comment to the contrary.

Highlights in this release include:

    The usual round of bug fixes, compatibility changes, and
    incremental improvements.

For a list of open issues, or to report problems, please visit
bugs.ghostscript.com.  Incompatible changes

    The planned device API tidy did not happen for this release,
    due to time pressures, but we still intend to undertake the
    following: We plan to somewhat tidy up the device API. We intend
    to remove deprecated device procs (methods/function pointers)
    and change the device API so every device proc takes a graphics
    state parameter (rather than the current scheme where only a
    very few procs take an imager state parameter). This should
    serve as notice to anyone maintaining a Ghostscript device
    outside the canonical source tree that you may (probably will)
    need to update your device(s) when these changes happen. Devices
    using only the non-deprecated procs should be trivial to update.