Allow X11 autolaunch to be disabled even if the headers/libraries are there

Allow X11 autolaunch to be disabled even if the headers/libraries are there

In an embedded system where the D-Bus session is a core part of the
environment, like Maemo, accidentally auto-launching a second session bus
(for instance for a concurrent ssh session) is a bad idea - it can lead
to a "split brain" situation where half the applications in the GUI are
using a different bus. In these controlled environments, it'd be useful
to prevent autolaunch from ever happening.

(As a side benefit, the changes to configure.in also mean that packagers
can explicitly --enable-x11-autolaunch, to make sure that failure to find
X will make compilation fail cleanly.)

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=19997
Bug-NB: NB#219964