fixed: libusb/libudev/libcec configure mess. logic is like this: libudev OR libusb...

fixed: libusb/libudev/libcec configure mess. logic is like this: libudev OR libusb is needed for usb device detection in XBMC under linux. if libudev is available, we don't need libusb and we prefer libudev, since it provides more information about the hardware (for the cec adapter, we need libudev in order to automatically detect the correct port. with libusb, users will have to enter the port path manually). because usb devices won't be detected when both libusb and libudev are not available, libcec will be useless, so it's disabled in that case.