In the interest of keeping this as simple as possible, I've extracted
the source files for the FXRuby User's Guide into a separate project
(fxruby-guide).

I added -I/usr/local/include to $CFLAGS and -L/usr/local/lib to
$LDFLAGS for the Darwin build, so that it would find the jpeg and
tiff libs on my system.

and -L/usr/X11/lib to $LDFLAGS when building on OS X. This will ensure that
it picks up the built-in PNG library.

Modified the config_options for the ExtensionTask to point to the full DOS path for FOX installation