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/libcore.cpp

@@ -310,5 +310,5 @@ const luaL_Reg corelib[] = {
     {"open_url", lua::wrap<l_open_url>},
     {"quit", lua::wrap<l_quit>},
     {"capture_output", lua::wrap<l_capture_output>},
-    {NULL, NULL}
+    {nullptr, nullptr}
 };