While disable by default, passing --enable-libinput-backend to
./configure switches the input backend in weston's drm, fbdev and rpi
compositing backends to use libinput instead of udev-seat.c, evdev.c and
friends.

When enabled, weston now also depends on libinput >= 0.1.0.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>