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/libutf8.cpp

@@ -99,5 +99,5 @@ const luaL_Reg utf8lib[] = {
     {"lower", lua::wrap<l_lower>},
     {"encode", lua::wrap<l_encode>},
     {"escape", lua::wrap<l_escape>},
-    {NULL, NULL}
+    {nullptr, nullptr}
 };