Add a section to mkfiles.pl to build a makefile that compiles the OS X

front end using GNUstep, yielding a Unix program built from the same
code. Should make it easier to check OS X behaviour without having to
move as far as a Mac :-) However, it doesn't compile as is, so I'll
apply fixes to the code until it does.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9498 cda61777-01e9-0310-a592-d414129be87e