> lang/python23/Makefile.common uses
[..]

> Is this something the other python versions should also get?

Yes. + the setup.py related patches to make it work..

> Furthermore and probably unrelated, I had to add
>
> .include "../../devel/zlib/buildlink3.mk"

Thanks, I've added it.

> I wonder whether this should or should not be implied by
> security/openssl/buildlink3.mk, which had already been included.

OpenSSL doesn't need zlib.