Don't invoke pkg-config at build time, automatically check
for sensorfw and compile the correct plugin if found.

Change-Id: I30dcd41cdebd7cf082cec66f3122c24fbab5c893
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>