Search notes:

/etc/portage/make.conf

/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.
#

TODO

# mirrorselect -i -o >> /mnt/gentoo/etc/portage/make.conf
# mirrorselect -i -o >>            /etc/portage/make.conf
$ emerge --info
$ emerge --info | grep '^CFLAGS='
$ emerge --info | grep '^USE='

Variables

ACCEPT_LICENSE
CFLAGS and CXXFLAGS
CHOST
CONFIG_PROTECT
CPU_FLAGS_*
DISTDIR
EMERGE_DEFAULT_OPTS
FEATURES
GENTOO_MIRRORS
INPUT_DEVICES
L10N
LINGUAS Used by some gettext-based build systems to control which localization files built and installed.
MAKEOPTS
PKGDIR
PORTAGE_SCHEDULING_POLICY
PORTAGE_TMPDIR
USE
USE_EXPAND
VIDEO_CARDS This variable expands into the USE_EPXAND variable. Multiple values can be separated by a space.
$ portageq envvar VIDEO_CARDS
  …

See also

man 5 make.conf
/usr/share/portage/config/make.conf.example

Links

https://wiki.gentoo.org/wiki//etc/portage/make.conf

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