Fix some unused-variable warnings.

A test-build with a modern clang points out a number of 'set but not
used' variables, which clang seems to have got better at recently.

In cases where there's conditioned-out or commented-out code using the
variable, I've left it in and added a warning-suppressing cast to
void. Otherwise I've just deleted the variables.

samegame.c

@@ -865,6 +865,8 @@ static void gen_grid(int w, int h, int nc, int *grid, random_state *rs)
 
 #if defined GENERATION_DIAGNOSTICS || defined COUNT_FAILURES
     printf("%d failures\n", failures);
+#else
+    (void)failures;
 #endif
 #ifdef GENERATION_DIAGNOSTICS
     {