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/libworld.cpp

@@ -251,5 +251,5 @@ const luaL_Reg worldlib[] = {
     {"save_chunk_data", lua::wrap<l_save_chunk_data>},
     {"count_chunks", lua::wrap<l_count_chunks>},
     {"reload_script", lua::wrap<l_reload_script>},
-    {NULL, NULL}
+    {nullptr, nullptr}
 };