anchor.diff:  
     # Hack to avoid TMPDIR in RPATHs.
     preFixup = ''rm -rf "$(pwd)" '';
  -  configureFlags = [ "--with-async-mode" ];
  +  configureFlags = lib.optional (!isDarwin) [ "--with-async-mode" ];
   
     # allow async mode. from ubuntu. see:
     #   https://bazaar.launchpad.net/~ubuntu-branches/ubuntu/trusty/libftdi/trusty/view/head:/debian/patches/04_async_mode.diff
  
positive.code: {stdenv, fetchurl, libusb}:
  
  stdenv.mkDerivation rec {
    name = "libftdi-0.20";
  
    src = fetchurl {
  
negative.code:   meta = {
      description = "A library to talk to FTDI chips using libusb";
      homepage = https://www.intra2net.com/en/developer/libftdi/;
      license = lib.licenses.lgpl21;
      platforms = lib.platforms.all;
    };
  }
  