texlive.combine: wrap with FONTCONFIG_FILE

This is neccessary for XeTeX to find the fonts distributed with
TeXLive.

The fontconfig file generated by makeFontsConf is almost identical to the one
from the fontconfig package and /etc/fonts/fonts.conf on NixOS -- it only adds
the specified font directories. In particular this means that all the actual
configuration files in /etc/fonts/conf.d are still included and there also is a
/usr/share/fonts font directory entry for use with non-NixOS linuxes, so
it's unlikely that this breaks someone's setup.

pkgs/tools/typesetting/tex/texlive/combine.nix

@@ -196,7 +196,11 @@ in (buildEnv {
       makeWrapper "$target" "$link" \
         --prefix PATH : "${gnused}/bin:${gnugrep}/bin:${coreutils}/bin:$out/bin:${perl}/bin" \
         --prefix PERL5LIB : "$PERL5LIB" \
-        --set-default TEXMFCNF "$TEXMFCNF"
+        --set-default TEXMFCNF "$TEXMFCNF" \
+        --set-default FONTCONFIG_FILE "${
+          # neccessary for XeTeX to find the fonts distributed with texlive
+          makeFontsConf { fontDirectories = [ "${texmfroot}/texmf-dist/fonts" ]; }
+        }"
 
       # avoid using non-nix shebang in $target by calling interpreter
       if [[ "$(head -c 2 "$target")" = "#!" ]]; then
@@ -311,5 +315,3 @@ in (buildEnv {
   ''
   ;
 }).overrideAttrs (_: { allowSubstitutes = true; })
-# TODO: make TeX fonts visible by fontconfig: it should be enough to install an appropriate file
-#       similarly, deal with xe(la)tex font visibility?

pkgs/tools/typesetting/tex/texlive/default.nix

@@ -5,7 +5,7 @@
 { stdenv, lib, fetchurl, runCommand, writeText, buildEnv
 , callPackage, ghostscript_headless, harfbuzz
 , makeWrapper, python3, ruby, perl, gnused, gnugrep, coreutils
-, libfaketime
+, libfaketime, makeFontsConf
 , useFixedHashes ? true
 , recurseIntoAttrs
 }:
@@ -24,7 +24,7 @@ let
   # function for creating a working environment from a set of TL packages
   combine = import ./combine.nix {
     inherit bin combinePkgs buildEnv lib makeWrapper writeText
-      stdenv python3 ruby perl gnused gnugrep coreutils libfaketime;
+      stdenv python3 ruby perl gnused gnugrep coreutils libfaketime makeFontsConf;
     ghostscript = ghostscript_headless;
   };