anchor.diff:        (mkFlag opusSupport "opus")
         (mkFlag soundcloudSupport "soundcloud")
         "--enable-debug"
  +      "--with-zeroconf=avahi"
       ]
       ++ opt stdenv.isLinux
         "--with-systemdsystemunitdir=$(out)/etc/systemd/system";
  
positive.code: , soundcloudSupport ? true, yajl
  }:
  
  let
    opt = stdenv.lib.optional;
    mkFlag = c: f: if c then "--enable-${f}" else "--disable-${f}";
  
negative.code:     ++ opt stdenv.isDarwin darwin.apple_sdk.frameworks.CoreAudioKit
      ++ opt stdenv.isLinux systemd
      ++ opt (stdenv.isLinux && alsaSupport) alsaLib
      ++ opt avahiSupport avahi
      ++ opt avahiSupport dbus
      ++ opt flacSupport flac
      ++ opt vorbisSupport libvorbis
      # using libmad to decode mp3 files on darwin is causing a segfault -- there
  