how to make tree view headers clickable

[Antonio Coralles <noche suapie reflex at>]

I would like to make the headers of some tree views clickable, so that
the user can sort their contents by the collumn header he clicks on,
like we see this for example in the gnome-sytem-monitor, or the
gtk-demo. However, i did not find out how to activate this behavour -
Gtk::TreeView::set_headers_clickable(...) didn't change anything for me
... I'm using gtkmm-2.6.3.
Thanks, Antonio