This macro expands to:

#define WinEnableMenuItem(hwndMenu, usId, fEnable)
((BOOL)WinSendMsg(hwndMenu,
                  MM_SETITEMATTR,
                  MPFROM2SHORT(usId, TRUE),
                  MPFROM2SHORT(MIA_DISABLED, (BOOL)(fEnable) ? 0 : MIA_DISABLED)))

This function requires the existence of a message queue.