Colin Watson suggests that Alt-click (or Option-click) could

usefully be equivalent to right-clicking on platforms other than OS
X; in particular, it's useful if you're running Linux on Apple
hardware such as PowerBook which inherently has only one button. So
here's the fix for GTK, and Windows as well (the latter for
completeness and consistency, not because I can actually think of
any reason somebody might be running Windows on one-button
hardware).

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5907 cda61777-01e9-0310-a592-d414129be87e