Check for X even if X11 autolaunching is disabled

Check for X even if X11 autolaunching is disabled

DBUS_ENABLE_X11_AUTOLAUNCH obviously requires DBUS_BUILD_X11. However,
the converse is not true.

If DBUS_BUILD_X11 is defined, dbus-launch will be able to connect to
the X server to determine when the session ends; most distributors will
want this, but it can be disabled with the standard Autoconf option
--without-x.

If DBUS_ENABLE_X11_AUTOLAUNCH is *also* defined, dbus-launch and libdbus
will be willing to perform autolaunch. Again, most distributors will want
this, but it can be disabled with --disable-x11-autolaunch.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=19997
Reviewed-by: Colin Walters <walters@verbum.org>