Respect the configure script run for udev

Configure does not currently have an argument for udev, but still: it will try
to detect it and if it is available QT_CONFIG contains the "libudev" word.

We should respect that for Qt 5. Unfortunate, the configure script is not
checking this for Qt 4, so we still need to fall back to pkgconfig for Qt 4.

Change-Id: I9e755ee015f4f584e945cb176f1788b6dbac7374
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>