... and it actually built directly.  :-)

Now, the widgets that use Pango for font rendering (I suppose), like
menus, display squares instead of actual characters, and the console is
filled with things like:

--8<---------------cut here---------------start------------->8---
Fontconfig error: line 139: invalid attribute 'xml:space'
Fontconfig error: line 140: invalid attribute 'xml:space'
Fontconfig error: line 146: invalid attribute 'xml:space'
Fontconfig warning: "/nix/store/i7gz1hrlrpvaz0dr3qj8hhnqx3rg60rn-fontconfig-2.8.0/etc/fonts/conf.d/30-metric-aliases.conf", line 84: Having multiple <family> in <alias> isn't supported and may not work as expected
Fontconfig warning: "/nix/store/i7gz1hrlrpvaz0dr3qj8hhnqx3rg60rn-fontconfig-2.8.0/etc/fonts/conf.d/30-metric-aliases.conf", line 84: Having multiple <family> in <alias> isn't supported and may not work as expected
Fontconfig warning: "/nix/store/i7gz1hrlrpvaz0dr3qj8hhnqx3rg60rn-fontconfig-2.8.0/etc/fonts/conf.d/30-metric-aliases.conf", line 93: Having multiple <family> in <alias> isn't supported and may not work as expected
--8<---------------cut here---------------end--------------->8---

I guess we’ll need a pseudo-package that creates a fonts.conf file for a
given set of fonts, like
<https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/libraries/fontconfig/make-fonts-conf.nix>.

Ludo’.