anchor.diff:  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
  
positive.code:     ++ optional sdl2Support        SDL2
      ++ optional cacaSupport        libcaca
      ++ optional vaapiSupport       libva
      ++ optional vapoursynthSupport vapoursynth
      ++ optionals dvdnavSupport     [ libdvdnav libdvdnav.libdvdread ]
      ++ optionals x11Support        [ libX11 libXext mesa libXxf86vm ]
  
negative.code: , libpngSupport      ? true,  libpng        ? null
  , youtubeSupport     ? true,  youtube-dl    ? null
  , vaapiSupport       ? true,  libva         ? null
  , drmSupport         ? true,  libdrm        ? null
  , vapoursynthSupport ? false, vapoursynth   ? null
  , jackaudioSupport   ? false, libjack2      ? null
  
  