Enable running on D-Bus system daemon

[Matt Hoosier]

Hi all,

I'm a fellow developer of Keith Preston, working with on an embedded
device which offers multiuser support. A system-level daemon provides
custom implementations of most of the Geoclue services.

Because (1) this process is run as a dedicated user and isn't attached
to any particular login session, and (2) Geoclue needs to still be
usable by numerous different user accounts, I've added a patch which
allows geoclue-master to be run on the system bus (see attachment).

Basically, we introduce a new switch (--enable-system-bus) which
mimics that of gconf-dbus, and then use the value indicated by that
when fetching the right DBusConnection at runtime.

Also note that I've added a new external method
"geoclue_get_dbus_bus_type()" which just exists to allow backend
services to query which of the two buses was specified at
configure-time, so that they'll supply the right connection when
attempting to register their backend with geoclue-master.

I've been using this successfully (both in session and system mode)
for several months now, without any ill effects.

Are there any objections to committing it?