The build system automagically determines if Emacs is present. Also, the elisp files are not installed in a proper subdir, but in site-lisp/ directly. Patch will follow.
Created attachment 163030 [details, diff] Proposed patch for guile-1.8.5,ebuild
Created attachment 163031 [details] Site-init file, files/50guile-gentoo.el
Fixed in guile-1.8.5-r1 (with permission from hkbst).