fix warnings (#648)

* fix unknown command line argument

* fix warnings settings for gcc

* fix -Wzero-as-null-pointer-constant

* revert CMakeLists.txt

src/logic/scripting/lua/libs/libitem.cpp

@@ -147,5 +147,5 @@ const luaL_Reg itemlib[] = {
     {"reload_script", lua::wrap<l_reload_script>},
     {"has_tag", lua::wrap<l_has_tag>},
     {"__get_tags", lua::wrap<l_get_tags>},
-    {NULL, NULL}
+    {nullptr, nullptr}
 };