Add libinput based input device backend

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>
diff --git a/src/compositor-fbdev.c b/src/compositor-fbdev.c
index f8d2fe0..e703e0e 100644
--- a/src/compositor-fbdev.c
+++ b/src/compositor-fbdev.c
@@ -42,7 +42,7 @@
 #include "compositor.h"
 #include "launcher-util.h"
 #include "pixman-renderer.h"
-#include "udev-seat.h"
+#include "udev-input.h"
 #include "gl-renderer.h"
 
 struct fbdev_compositor {