qt: enable proper re-configure

The Qt configure script doesn't use config.cache but the
generated Makefile provides a confclean target to properly
clean out old configuration.

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>