Fix Qt build when mtdev is detected

If it's detected, we have "mtdev" in QT_CONFIG, not in CONFIG. With
the bad test, libQt5PlatformSupport.prl would not get -lmtdev and, in
turn, the evdevtouch generic plugin would fail to link.

Change-Id: I5dab57b648e66943f98a22527717a20be35f02a4
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Fix mtdev auto-detection

The source code is valid C++. But not valid C.

Change-Id: I43126001d33caef050b0bc8c248cf9f3b01091df
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>