Some updates to the installation instructions

Mention X11 build requirements (since XInput2 is something of
an actual requirement, as opposed to 'newer than R5').

INSTALL.in

@@ -9,6 +9,10 @@ GTK+ requires the following packages:
    GdkPixbuf @GDK_PIXBUF_REQUIRED_VERSION@, ATK @ATK_REQUIRED_VERSION@
    and cairo @CAIRO_REQUIRED_VERSION@.
 
+ - Each GDK backend has its own backend-specific requirements. For
+   the X11 backend, X11 R6 and XInput version 2 (as well as a number
+   of other extensions) are required.
+
  - gobject-introspection @INTROSPECTION_REQUIRED_VERSION@ or newer.
 
 Simple install procedure