dbus-glib: tighten dbus expat dependency

Depend on dbus being compiled with expat support and not just on expat
being enabled, as you can have expat enabled and still compile dbus
with libxml2 support instead.

While we're at it, only show comment about unforfilled dependencies if
dbus is enabled.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>