Emerging with a custom make option?

[spacehaven]

I was wondering if it's possible to emerge a package, and have the package built with a custom ./Configuration option.

Specifically, I installed skey, and I want to rebuild openssh to include skeu functionality, but I don't know how to pass '-with-skey' to openssh's ./Configure. Sure, I could easily download openssh-foo.tar.gz, but I want to try and stay within the Gentoo/portage way of doing things.

It would seem likely it would or should be in an etc make.conf file or similar, so that if I were to do a later 'emerge update openssh', the configuration option would be reused. But I couldn't find any info about this in the portage user guide, the manual, the man pages, or anywhere.

Thanks!