Related bug: #122236 To work with C++ I had to create overlay ebuild of dev-lisp/ecls (both 13.5.1-r1 and 16.0.0), it sufficed only to add '--with-cxx' to econf, and it works fine. Perhaps it can be enabled by default, as it only depends on C++ compiler.
commit 8afadd064d413c088d88d34845fd500b2c00c698 Author: Andrey Grozin <grozin@gentoo.org> Date: Wed Mar 2 14:18:33 2016 +0600 dev-lisp/ecls: add C++ support to 16.0.0 and 16.1.2 Bug: 576048 Package-Manager: portage-2.2.27