Version 2.7.1 is available upstream. Please consider updating!
It seems that version 2.7.1 is available upstream, while the latest version in the Gentoo tree is 2.6.0. You think this warning is false? Read more about it here.
libint is two things:
1. a library of C/C++ functions for efficient evaluation of several
kinds of two-body molecular integrals over Gaussian functions;
2. the optimizing compiler that generates a Libint library.