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.

[originally from svn r7021]

no-icon.c

@@ -0,0 +1,9 @@
+#include <X11/Intrinsic.h>
+
+/*
+ * Dummy source file which replaces the files generated in the
+ * `icons' subdirectory, when they're absent.
+ */
+
+const char *const *const xpm_icons[] = { 0 };
+const int n_xpm_icons = 0;