NixOS does not correctly compile gxml
!24 (merged) fixes this issue.
I'm currently packaging gxml into Nixpkgs, and it will not work unless I use this PR commit. The issue is that when compiling a program that uses gxml, it's checking the /include folder in gxml-0.20/include, but in Nix it should be checking it in gxml-0.20-dev/include. This fixes the problem.