anchor.diff:  assert rubberbandSupport  -> available rubberband;
   assert screenSaverSupport -> available libXScrnSaver;
   assert sdl2Support        -> available SDL2;
  +assert sixelSupport       -> available libsixel;
   assert speexSupport       -> available speex;
   assert theoraSupport      -> available libtheora;
   assert vaapiSupport       -> available libva;
  
positive.code:     ++ optional rubberbandSupport  rubberband
      ++ optional screenSaverSupport libXScrnSaver
      ++ optional sdl2Support        SDL2
      ++ optional speexSupport       speex
      ++ optional theoraSupport      libtheora
      ++ optional vaapiSupport       libva
  
negative.code:     (enableFeature dvdnavSupport   "dvdnav")
      (enableFeature openalSupport   "openal")
      (enableFeature sdl2Support     "sdl2")
      (enableFeature sixelSupport    "sixel")
      (enableFeature vaapiSupport    "vaapi")
      (enableFeature waylandSupport  "wayland")
      (enableFeature stdenv.isLinux  "dvbin")
  