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.

unruly.c

@@ -1398,6 +1398,8 @@ static char *new_game_desc(const game_params *params, random_state *rs,
             temp_verbose = solver_verbose;
             solver_verbose = false;
         }
+#else
+        (void)attempts;
 #endif
 
         unruly_free_scratch(scratch);