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/libapp.cpp

@@ -2,5 +2,5 @@
 
 const luaL_Reg applib[] = {
     // see libcore.cpp an stdlib.lua
-    {NULL, NULL}
+    {nullptr, nullptr}
 };