gtkmm3?

[Josh Bialkowski <jbialk MIT EDU>]

I was reading the gtkmm book and when I hit the clock example for
drawing widgets with cairo, I realized that this was exactly what I was
looking for (i.e. wanting to draw widgets with cairo). It was only after
I couldn't get the example to compile that I noticed the note that it
was for gtkmm3 not gtkmm2 (after all, I guess gtk3 is where all the
cairo drawing is at anyway). Anyway, so my question is, where is gtkmm3?
I downloaded gtkmm-2.90.7, 2.91.7, and 2.99.5 guessing that these might
be develoment snapshots and gtkmm3 doesn't actually exist yet. They're
promising because the configure script seems to require gtk3, but I
can't seem to build any of them. 

Anyway, so is gtkmm3 without a stable branch at this moment? If so, how
stable are the unstable branches? Should I be able to build and use
them? Or is the book just ahead of the game?