patch - do not dist cocoa sources

Hi,

as discussed here is a patch to not ship the unmaintained cocoa
sources in the tarballs and build abiword-gtk by default when platform
`darwin' is detected. This yields 0.2M of tarball size. Restoring the
prior behaviour is as easy as changing two lines in configure.in, in
case interest in the port lives up again.

The OSX rendering problems have neither been fixed during the 2.4 nor
the 2.6 cycle, so IMO it is fair enough not to inflict the
unmaintained parts on tarball downloaders.

Ok to commit?

- Rob