ThickClient: avoid using GConf workaround for --disable-gnome (bgo#692374)

Previous fix to make dbus-glib-1 dependency mandatory [1] when still
using GConf missed the fact that Mac and Windows builds are compiled
with --disable-gnome flag, and the workaround shouldn't be called in
these cases.

[1] https://git.gnome.org/browse/banshee/commit/?id=abdfd9c0786d159b7c24cbcff56eabd61f5d6b9a