Install this version:
emerge -a =sys-libs/minilog-9999
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =sys-libs/minilog-9999
Or alternatively:
emerge --autounmask-write -a =sys-libs/minilog-9999
# Copyright 1999-2022 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2
EAPI=8
inherit git-r3
DESCRIPTION="A minimal header only C++ logger system"
HOMEPAGE="https://github.com/dominikschnitzer/minilog"
EGIT_REPO_URI="https://github.com/dominikschnitzer/minilog.git"
LICENSE="BSD2"
SLOT="0"
KEYWORDS="~amd64 ~x86"
IUSE=""
RDEPEND=""
DEPEND="${RDEPEND}"
src_install() {
insinto /usr/include/minilog
doins minilog.h
}