This needs a real Glk implementation to run.   We can either have
the user do this and note it in MESSAGE, or depend on one.  If we
decide to depend on one, we can either depend on glkterm, which
pulls in ncurses on NetBSD 1.6 and below, or create a package for
cheapglk, which probably no one will use.  For now, it depends on
glkterm.