From maxima-5.17.1-r1, it does not install imaxima; instead, it depends on separated app-emacs/imaxima package, so please add it to the prefix tree. I tested it by ecopying at first, but it seems to need hand editing: the line econf --with-lispdir="${SITELISP}/${PN}" || die "econf failed" to econf --with-lispdir="${EPREFIX}/${SITELISP}/${PN}" || die "econf failed" With that, emerge succeeds and installed imaxima works fine.
added, thanks!