Support for run-time icons in the GTK puzzles. This involved another

mkfiles.pl change (I don't seem to be planning ahead very well this
week), this time to provide a list of fallback options for an object
file. That way, I have a no-icon.c which quietly replaces
icons/foo-icon.c if the latter doesn't exist, and so again people
checking straight out from Subversion shouldn't have trouble.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@7021 cda61777-01e9-0310-a592-d414129be87e