anchor.diff:      ++ optional sdl2Support        SDL2
       ++ optional cacaSupport        libcaca
       ++ optional vaapiSupport       libva
  +    ++ optional drmSupport         libdrm
       ++ optional vapoursynthSupport vapoursynth
       ++ optionals dvdnavSupport     [ libdvdnav libdvdnav.libdvdread ]
       ++ optionals x11Support        [ libX11 libXext mesa libXxf86vm ]
  
positive.code: , libpngSupport      ? true,  libpng        ? null
  , youtubeSupport     ? true,  youtube-dl    ? null
  , vaapiSupport       ? true,  libva         ? null
  , vapoursynthSupport ? false, vapoursynth   ? null
  , jackaudioSupport   ? false, libjack2      ? null
  
  
negative.code: assert vapoursynthSupport -> available vapoursynth;
  assert jackaudioSupport   -> available libjack2;
  assert vaapiSupport       -> available libva;
  assert drmSupport         -> available libdrm;
  
  let
    # Purity: Waf is normally downloaded by bootstrap.py, but
  