Link dbus-daemon and dbus-daemon-lauch-helper against libdbus

Link dbus-daemon and dbus-daemon-lauch-helper against libdbus

The shared can be used by dbus-daemon and dbus-daemon-launch-helper by exporting
the private symbols needed, reducing the size of dbus by about 500k.
The private symbols are exposed under the version
LIBDBUS_PRIVATE_@VERSION_NUMBER@.

[Altered by Simon McVittie and Ralf Habacker to clear up some
problematic linking.]

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=83115
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>