First: git clone this to /usr/share/nilcons-fonts.

Then put this into your X session init file (or bashrc, as you wish):

NILCONS_FONTS=/usr/share/nilcons-fonts
[ -d $NILCONS_FONTS ] && {
  # Use only nilcons-fonts for fontconfig
  export FONTCONFIG_FILE=ncfonts.conf
  export FONTCONFIG_PATH=$NILCONS_FONTS

  # Control the traditional X fonts in the same way
  xset fp= $NILCONS_FONTS/xfonts,built-ins

  [ -e $NILCONS_FONTS/user/xfonts/fonts.dir ] && xset +fp $NILCONS_FONTS/user/xfonts
}

(We have to use /usr/share/<something>, because there are a lot of VERY-VERY hard to debug sandboxing measures in GNU/Linux nowadays. E.g. AppArmor is kind of easy, but Firefox sandbox is a nightmare. Random complicated apps will not work if you try somewhere else. See: https://github.com/mozilla/gecko-dev/blob/master/security/sandbox/linux/broker/SandboxBrokerPolicyFactory.cpp)

Fontconfig 2.12 (nix @2019-01-21):

• understands FONTCONFIG_{FILE,PATH}
• with the above config only reads /usr/share/nilcons-fonts/*
• doesn't understand FONTCONFIG_SYSDIR
• works if proper configuration given at /usr/share/nilcons-fonts/ncfonts.conf

Fontconfig 2.13 (debian @2019-01-21):

• would understood FONTCONFIG_SYSDIR
• reads /usr/share/fontconfig no matter what (even with FONTCONFIG_SYSDIR)
• fortunately /usr/share/fontconfig is only parsed, not loaded, and merge request is already submitted to fix this (https://gitlab.freedesktop.org/fontconfig/fontconfig/merge_requests/26)
• otherwise only reads /usr/share/nilcons-fonts/*
• works if proper configuration supplied there

Use cases for this knowledge:

• run your system with only the 3 liberation fonts + Iosevka, should be enough for everything
• no Debian/Ubuntu/Arch font stupidity to deal with
• compatibility with 2.12 AND 2.13 fontconfig libraries and static binaries at the same time
• test if your website uses any fonts which are not provided by you (this needs further research, we need a config, where only a font with all boxes are provided, then chrome will have some UI and user can test)
• test your application's font dependence
• package a GNU/Linux application together with its fonts

TODO:

• add a directory where user can drop fonts
• add a directory where user can drop fontconfigs
• these dirs should be gitignored
• test these!

Useful commands:

• fc-list
• fc-list : file
• fc-cache -fv
• fc-conflist (you can ignore lines that start with '-')
• FC_DEBUG=4 fc-match 'mono,sans,serif'
• pango-list
• pango-view --font '' --text 'foobar quux'