This reworks commit 20e1508e, for two
reasons:
 - Upstream dbus.git now does the same (although this isn’t yet reflected
   in the online version of the D-Bus Specification); see
   https://gitlab.freedesktop.org/dbus/dbus/-/merge_requests/209.
 - It allows local-prefix (e.g. jhbuild) builds of GLib to build in a
   custom prefix while still interacting with system services using the
   system-wide `/run` directory. To do so, pass `-Druntime_dir=/run` to
   meson configure.

As documented in the `NEWS` file in
https://gitlab.freedesktop.org/dbus/dbus/-/merge_requests/209, it’s only
valid to use `/run` – rather than `/var/run` – for D-Bus if the two
paths are interoperable. i.e. `/var/run` should be a symlink to `/run`,
and the D-Bus daemon should be configured to put its socket there.

This commit deliberately doesn’t introduce a special `system_socket`
configure option for specifying where the D-Bus system socket lives, as
that would only be useful for a distribution which sets `runstatedir` to
something other than `/var/run` or `/run`, which seems unlikely. We
could add such an option in future, though, if a distribution comes
forward with such a requirement.

See discussion on
GNOME/glib!3095 (comment 1605502)

.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>