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/libparticles.cpp

@@ -82,5 +82,5 @@ const luaL_Reg particleslib[] = {
     {"is_alive", wrap_hud<l_is_alive>},
     {"get_origin", wrap_hud<l_get_origin>},
     {"set_origin", wrap_hud<l_set_origin>},
-    {NULL, NULL}
+    {nullptr, nullptr}
 };