Ghostscript: Add '--with-djvu' option to build out-of-tree DjVU format drivers

The gsdjvu GS drivers were released by AT&T under a FOSS license that is incompatible with Ghostscript and therefore cannot be distributed as part of the Ghostscript source code or binaries. gsdjvu's own build/installation script is clunky, it is much nicer for the user if the ghostscript formula is patched to build gsdjvu.

Tested on Mountain Lion. I did some conversions of scanner-created PDFs and of digitally-authored PDFs, no issues.

tldr:
DjVU is a format for compressed scans of text documents and any included images. It achieves better compression and faster rendering than PDF versions of the same content. DjVULibre, a free implementation for reading and writing djvu files, installs the conversion script 'djvudigital' which utilizes Ghostscript and the external 'gsdjvu' drivers to convert PDFs and other common scanned products into DjVU.