As per summary, jack-rack fails to build with GTK+ 2.12. I'm opening this bug report as a reminder for myself, if somebody else don't want to take care of it before I can return. There's a new 1.4.6 release, that one might work with GTK+ 2.12, has to be tried first. Otherwise leio suggests to remove GTK_DISABLE_DEPRECATED.
Fixed now in 1.4.6, we need to remember to mark it stable when Gtk 2.12 is going to.