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.

magnets.c

@@ -1590,6 +1590,7 @@ static int lay_dominoes(game_state *state, random_state *rs, int *scratch)
     }
 
     debug(("Laid %d dominoes, total %d dominoes.\n", nlaid, state->wh/2));
+    (void)nlaid;
     game_debug(state, "Final layout");
     return ret;
 }