Fl_Double_Window *wnd_config = 0;

// DLL entry
BOOL APIENTRY DllMain(HANDLE hmodule, DWORD ul_reason_for_call, LPVOID lpReserved)
{
	switch(ul_reason_for_call)
	{
		case DLL_PROCESS_ATTACH:
		case DLL_THREAD_ATTACH:
		case DLL_THREAD_DETACH:
		case DLL_PROCESS_DETACH:
				break;
	}
	return TRUE;
}

Fl_Double_Window *showGUI()
{
	wnd_config = make_window();
	return wnd_config;
}

//... some includes...

#define DLLExport extern "C" __declspec(dllexport)
DLLExport Fl_Double_Window *showGUI();

void btn_system_cb(class Fl_Button *,void *)
{
	Fl_Double_Window *wnd;

        // showGUI() is the imported DLL function from dll.dll
        // the DLL (dll.dll) is loaded dynamically with LoadLibrary and GetProcAddress etc.
	wnd = showGUI();
        wnd->show();
}