Version 2.13.1 is available upstream. Please consider updating! It seems that version 2.13.1 is available upstream, while the latest version in the Gentoo tree is 2.11.0-r1. 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.