default.nix

let
  pkgs = import (builtins.fetchTarball {
    url = https://github.com/NixOS/nixpkgs/archive/a80ee5b697749d48422f4af6143d21d28cb5c022.tar.gz;
    sha256 = "0qbwairrj1yjg83ppfa9li18qlzzvr0h7b6w0i80hkc0phfb9z2x";
  }) {};
in

### echo $PKG_CONFIG_PATH
### /nix/store/mgzvrln0qd50r226cb3bf6v3xw2sd2q5-zlib-1.2.11-dev/lib/pkgconfig
# pkgs.stdenv.mkDerivation {
### echo $PKG_CONFIG_PATH
### no output
pkgs.python37Packages.buildPythonPackage {
  name = "example";
  src = ./.;
  buildPhase = ''
echo PKG_CONFIG_PATH=$PKG_CONFIG_PATH
'';
  nativePropagatedBuildInputs = [ pkgs.pkg-config pkgs.zlib ];
}