ghostscript: mostly rewrite recipe

This started as a patch cleanup but escalated rapidly.

Remove unneeded patches:
- mkdir-p.patch isn't needed now the Makefiles appear to have the correct
  dependencies.
- ghostscript-9.15-parallel-make.patch appears to be unneeded for the same
  reason
- base-genht.c-add-a-preprocessor-define-to-allow-fope.patch isn't needed
- cups-no-gcrypt.patch isn't needed
- do-not-check-local-libpng-source.patch can be replaced by deleting
  the libpng/ directory, as is already done for jpeg/
- ghostscript-9.21-native-fix-disable-system-libtiff.patch is not needed
  when we stop doing native builds (see below)

Remove the need for ghostscript-native to build and install tools that
are needed at target build-time: ghostscript can do this itself.  Remove
the BBCLASSEXTEND and all of the native overrides.

Inherit pkgconfig and explicitly tell configure to use the pkgconfig
binary: unless told otherwise this configure will refuse to use an
unprefixed pkgconfig in cross builds.

Review DEPENDS and add missing freetype and zlib dependencies.

Ghostcript will use the embedded copies of libraries over system
libraries, so extend the deletion of jpeg and libpng to include expat,
freetype, and cups as we want to link to our build of those. We can't
delete zlib as it is explicitly used when building the native tools.

Add PACKAGECONFIGs for optional libidn and libpaper dependencies.

Remove HAVE_SYS_TIME_H assignments, the upstream bug was fixed in 2011.

Clean up comments: there's no need to explain how to use PACKAGECONFIG,
and justify the use of autotools-brokensep.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>