pugixml: Fix pkg-config file installation and use fetchFromGitHub [19.03]

[tokudan]

Motivation for this change

Changes were merged into master after branch-off, this fixes the filezilla issues.
pugixml: Fix pkg-config file installation and use fetchFromGitHub

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option 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/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[tokudan]

filezilla builds, but fails to execute with a SIGSEGV. That's probably a conflict on my system, but I'd prefer if someone else could verify this.

$ LANG=C /nix/store/plac3bfdrqrmwz5ywlg6rdxwfq2pig63-filezilla-3.31.0/bin/filezilla
Reading locale option from /home/dfrank/.config/filezilla/filezilla.xml

(filezilla:10342): Gtk-WARNING **: 19:03:54.066: Unable to locate theme engine in module_path: "adwaita",

(filezilla:10342): Gtk-WARNING **: 19:03:54.070: Unable to locate theme engine in module_path: "adwaita",
Segmentation fault

[jokogr]

I've applied this to release-19.03 and it was properly working for me, I've managed even to resume a transfer and session manager was working.

@tokudan Could you maybe start this with an empty .config/filezilla?

[tokudan]

@tokudan Could you maybe start this with an empty .config/filezilla?

I already tried that. I guess it's a conflict with my 18.09 installation or my specific Gnome/GTK configuration.

Since it works for @jokogr and since it's been approved by pSub, I guess it's ready for #56826 to fix the Filezilla failure.