configure: disable ConsoleKit support by default

This commit requires users to explicitly --with-console-kit

It also adds a message pointing out ConsoleKit is deprecated and
links to a wiki page with more information. See:

https://mail.gnome.org/archives/desktop-devel-list/2015-January/msg00108.html

for discussion. A future commit will remove ConsoleKit support entirely.

https://bugzilla.gnome.org/show_bug.cgi?id=743940