gtk-demo and "foreigndrawing"

[Hans L <thehans gmail com>]

I've attempted to build gtkmm (3.22.3)  from source on Linux Mint 19,
but the Makefile under demos says there's "nothing to do for 'all' "

I'm mainly interested to see an example analogous to the
"foreigndrawing.c" code from GTK source here:
https://gitlab.gnome.org/GNOME/gtk/blob/3.22.30/demos/gtk-demo/foreigndrawing.c

But it seems that this is not included in the gtkmm demo code anyways.
I can find no mention of "foreign" at all in the source tree.

Does the gtkmm/demos/gtk-demo code really build / function or is it unfinished?
Any help would be appreciated.

- Hans Loeblich