gtk3 is buggy. It tries to link against installed libs during the
build, instead of only the libs being built.
I did
pkg_delete -f gtk3+
and then it builds fine.
Actually fixing this is harder; you'll have to find where gtk3+'s build
manages to have /usr/pkg/lib in the search path first. This sort of
thing is gradually getting better, but it's hard. And, people that only
pbulk don't hit it, so the set of motivated people is smaller. However,
it will probably only take one person who is both motivated and has a
round tuit to fix this.