qt: use configure option instead a sed hack to disable xinerama

The Qt configure script gained the option '-no-xinerama' in the
meantime. So use it instead hacking the script itself using sed.

Signed-off-by: Michael Roth <mroth@nessie.de>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>