[gentoo-dev] crossdev: installing _host_ build dependencies not automatic?

I encountered <https://bugs.gentoo.org/show_bug.cgi?id=617276> while
working on some cross-compiling project.

Admittedly, it may not be that easy to handle host package dependencies
fully automatically.

I'm wondering - is it documented what portage guarantees, and what I'm
expected to just manually handle to provide host build dependencies?

Any other advice about properly using crossdev would also be
appreciated. I'd be happy to test and help improve things.

Paweł