The postPatch phase of the fontconfig expression performs patching of
src/fcobjshash.{gperf,h} files and performs "touch" to ensure the make
rule will not get triggered. Editing files in the right order seems to
be a more robust way to do both tasks.

Motivation for this change

This is basically a black magic to fix the build work when $TMP is on a network file system.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.