Make the code base clean under -Wwrite-strings.

I've also added that warning option and -Werror to the build script,
so that I'll find out if I break this property in future.

sixteen.c

@@ -398,8 +398,7 @@ static char *new_game_desc(const game_params *params, random_state *rs,
 
 static const char *validate_desc(const game_params *params, const char *desc)
 {
-    const char *p;
-    char *err;
+    const char *p, *err;
     int i, area;
     int *used;