$ cat shell.nix
{ pkgs ? import <nixpkgs> {} }:

pkgs.mkShell
  { buildInputs = [
        pkgs.clang
        pkgs.binutils
        pkgs.fftwSinglePrec
        pkgs.llvmPackages.openmp
        pkgs.libfvad
    ];
  }

$ cat derivations/default.nix
self: super:

{
  libfvad = super.callPackage ./libfvad { };
}

$ cat derivations/libfvad/default.nix
{ pkgs
, autoreconfHook }:

let
  commit = "ee69951e7ec6ed3a0caf547f35f103280b9831de";
  stdenv = pkgs.stdenv;
in

stdenv.mkDerivation {
  name = "libfvad";
  src = pkgs.fetchgit {
    url = "https://github.com/talonvoice/libfvad.git";
    rev = commit;
    sha256 = "0jzwjx0ca6m1c422q54k1gw4y5ik18zcf9rx9bsbnbkgbmgh1agp";
  };
  enableParallelBuilding = true;

  meta = with stdenv.lib; {
    description = "A fork of the VAD engine that is part of the WebRTC Native Code package (https://webrtc.org/native-code/), for use as a standalone library independent from the rest of the WebRTC code.";
    homepage = "https://github.com/talonvoice/libfvad";
  };
}

$ nix-shell -I nixpkg-overlays=$(pwd)/derivations
error: attribute 'libfvad' missing, at <snip>/shell.nix:9:9
(use '--show-trace' to show detailed location information)