default.nix

with import <nixpkgs> {};
let
  callPackage = newScope self;
  self = {
    foo = callPackage ./foo.nix {};
    other = callPackage ./other.nix {};
  };
in self

foo.nix

{
someVar ? false
, stdenv, fetchurl, something, somethingelse, perl
}:

with stdenv.lib;

let
	version = "2.0";

in stdenv.mkDerivation rec {
	name = "foo-${version}";

	src = fetchurl {
		# ...
	};

	buildInputs = [ something ]
		++ optional (stdenv.isLinux || stdenv.isFreeBSD) somethingelse;

	nativeBuildInputs = [ perl ];

	configureFlags = []
		++ optional someVar "--some-setting"
	;

	enableParallelBuilding = true;

	meta = {
		# ...
	};
}

other.nix

{ stdenv, foo }:

stdenv.mkDerivation rec {
	...

	buildInputs = [ foo ];

	configureFlags = [
		"VAR=${foo}/bin/somebin"
	];

	...
}