community/mupdf: minor cleanup/refactor

Took over maintainership recently and finally had the chance to give it some love:

• Make sure it's compiled with our optimization set in /etc/abuild.conf
  • Results in package/binary size decrease
  • Previously used -O2 instead of -Os
• Set up the libmupdf.so symlinks in the order described in ldconfig(8)
• Minor (mostly cosmetic) cleanup of shared-lib.patch