configure: Use the system PKG_CONFIG_PATH

[cdecker]

I noticed this while working on a nix devShell to work on CLN. We are
blanked overriding the pkg-config search path, which can cause some
trouble. Specifically nix uses content addressable locations, and
macOS arm64 and x84_64 use separate pkg-config search paths, and by
overwriting it we can cause a mix of different architectures failing
the compilation

Changelog-Fixed: configure: We now respect the PKG_CONFIG_PATH environment variable

[vincenzopalazzo]

Post-review ACK 758c78e

Nice catch, I was wondering are you working on a nix dev shell and make a PR or it is just your things?

Asking this because if you are playing with it maybe I will not spend some time on this #6739 😸

[cdecker]

Indeed, my current flake.nix is the following:

{
  inputs = {
    nixpkgs.url = "nixpkgs/nixos-23.11";
    flake-utils.url = "github:numtide/flake-utils";
    rust-overlay.url = "github:oxalica/rust-overlay";
  };
  outputs = { self, nixpkgs, flake-utils, rust-overlay }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        overlays = [ (import rust-overlay) ];
        pkgs = import nixpkgs { inherit system overlays; };

        nativeBuildInputs = with pkgs; [ pkg-config sqlite ];
        buildInputs = with pkgs; [ sqlite rust-bin.stable.latest.default];
      in {
        devShells.default = pkgs.mkShell {
          packages = with pkgs; [
            autoconf
            automake
            bitcoind
            clang
            libtool
            git
            openssl
            pkg-config
            python310
            sqlite
            which
            zlib
          ];
          inherit nativeBuildInputs buildInputs;
          shellHook =
            ''
            export PATH=.venv/bin:$PATH
            if test -d .venv; then
              echo "Updating existing virtualenv in .venv"
              poetry update --with=dev
            else
              echo "Creating new virtualenv in .venv"
              python3 -m venv .venv || true
              python3 -m pip install -U wheel pip poetry
              poetry install --with=dev --no-root
            fi
            '';
          LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath [ pkgs.stdenv.cc.cc ];
        };
      });
}

It's somehow missing the cryptography python package for some tests (could just be the test using an absolute path), otherwise it can compile and run tests.