$ cat /etc/portage/make.conf
CC="clang"
CXX="clang++"

CFLAGS="-O2 -march=native"
CHOST="x86_64-pc-linux-gnu"

USE="-bindist -fortran -ipv6 -qt4 icu jpeg2k opencl opengl openssl vdpau"
CPU_FLAGS_X86="mmx mmxext popcnt sse sse2 sse3 sse4_1 sse4_2 ssse3"

ABI_X86="32 64"

MAKEOPTS="-j4"

PHP_TARGETS="php7-0"

ACCEPT_KEYWORDS="~amd64"

INPUT_DEVICES="libinput"
VIDEO_CARDS="nouveau"

PORTDIR="/usr/portage"
DISTDIR="${PORTDIR}/distfiles"
PKGDIR="${PORTDIR}/packages"

PORTDIR_OVERLAY=/usr/local/portage

FEATURES="ccache"
CCACHE_SIZE="4G"

======

$ cat /etc/portage/env/gcc
CC="gcc"
CXX="g++"

======

$ cat /etc/portage/package.env
=dev-lang/spidermonkey-1.8.5-r6     gcc
=dev-libs/elfutils-0.168            gcc
=dev-libs/libinput-1.7.2            gcc
=media-libs/id3lib-3.8.3-r8         gcc
=media-libs/libomxil-bellagio-0.9.3 gcc
=media-sound/audacity-2.0.5-r1      gcc
=net-wireless/crda-3.18-r1          gcc
=sys-boot/efibootmgr-15             gcc
=sys-devel/m4-1.4.18                gcc
=dev-db/mysql-5.6.36                gcc
=x11-libs/wxGTK-2.8.12.1-r2         gcc