/etc/portage/make.conf is the main Portage configuration file. It contains variables for the build-process and overwrites those in make.globals (/usr/share/portage/config/make.globals?)
The default values for most of these variables are set by a profile.
Among others, /etc/portage/make.conf defines the optimization flags for the compiler.
COMMON_FLAGS="-march native -O2 -pipe"
CFLAGS="${COMMON_FLAGS}"
CXXFLAGS="${COMMON_FLAGS}"
FCFLAGS="${COMMON_FLAGS}"
FFLAGS="${COMMON_FLAGS}"
# This sets the language of build output to English.
# Please keep this setting intact when reporting bugs.
LC_MESSAGES=C.utf8
#
# See also /etc/portage/repos.conf/gentoo.conf
#
GENTOO_MIRRORS="https://mirror.init7.net/gentoo/"
#
# Values defined with USE= are added to the system's USE flag list.
# In order to globally remove an item, it can be prefixed with a minus:
# Se also /var/db/repos/gentoo/profiles/use.desc
#
USE=-gtk -gnome qt5 kde dvd alsa cdr"
# ACCEPT_LICENSE="-* @FREE @BINARY-REDISTRIBUTABLE"
#
# Accept all licenses except EULAs:
# ACCEPT_LICENSE="* -@EULA"
#
#
# The @ sign identifies a license group.
#
# Try also: portageq envvar ACCEPT_LICENSE
# See also: /etc/portage/package.license and /var/db/repos/gentoo/profiles/license_group, GLEP 23.
#