Check all the drivers for one that'll drive our device in
question, and prefer ones that have a discover() method that runs
successfully to a driver without such a method.
This allow drivers to both reject devices (if 2 drivers handle
the same USB IDs), or a single driver to handle variants of the
same device (through the devtype out value of the discover method).
When imaging support is required, we prefer to use gdk-pixbuf,
as it's already on things like Live CDs.
Also fix the examples building against the system libfprint.
Bastien Nocera