Add a new option URWFONTS

This options allows to disable at build time dependency on gsfonts and
ghostcript, to reduce the number of dependency for minimal setup.

Note that ghostscript and gsfonts are only needed at build time otherwise (mark
ghostscritp as such in any case now) and they are needed for some usage of
gropdf.

Reported by: Zeus Panchenko <zeus@ibs.dn.ua>