You have to run the PHP script somewhere in the WDL sources to turn the Windows resource files into source code for building on Mac Os. That should produce a header file you must include when building for Mac Os. There was something else that needed to be done too in the #ifdef block but I don't remember exactly. The Reaper extension SDK example should have the correct way to write that...
edit : code, which maybe has to be put after the plugin entry point function.
Code:
#ifndef _WIN32 // MAC resources
#include "../../WDL/swell/swell-dlggen.h"
#include "res.rc_mac_dlg"
#undef BEGIN
#undef END
#include "../../WDL/swell/swell-menugen.h"
#include "res.rc_mac_menu"
#endif