Add --with-rundir configure flag

I'm not sure this is the best way to do it; I'll look more at
autotools docs later and fixup if needed, but this is just an
early attempt to make it work...

TODO:
  tools/70-udev-acl.rules (hardcoded /var/run/ConsoleKit)
  doc/console-kit-daemon.1m (hardcoded /var/run/ConsoleKit)