There are some packages (e.g. gnutls-3.5.9) that refer to libidn2 package config. Change that adds libidn2.pc is available in upstream repository, but not yet included in any version: https://gitlab.com/jas/libidn2/commit/84b396c528dfed9ea6e8248f0522d120c9bd6186.diff This issue, together with bug #609180 trigger configuration failure for any package that depends on gnutls, that built with libidn2 already in system (in my case wget configuration failed after gnutls update to 3.5.9).
commit 31d5415bc400feba9b5f82354aa1667a81f831ea Author: Lars Wendler <polynomial-c@gentoo.org> Date: Mon Feb 13 09:10:15 2017 net-dns/libidn2: Revbump to add pkgconfig patch (bug #609188). This is required for at least gnutls-3.5.9 and all packages depending on gnutls. Package-Manager: Portage-2.3.3, Repoman-2.3.1