anchor.diff:  
     NIX_LDFLAGS = with stdenv.lib;
          optional  fontconfigSupport "-lfontconfig"
  +    ++ optional  fribidiSupport "-lfribidi"
       ++ optionals x11Support [ "-lX11" "-lXext" ]
       ;
   
  
positive.code: 
  assert fontconfigSupport -> (fontconfig != null);
  assert (!fontconfigSupport) -> (freefont_ttf != null);
  assert x11Support -> (libX11 != null && libXext != null && mesa != null);
  assert xineramaSupport -> (libXinerama != null && x11Support);
  assert xvSupport -> (libXv != null && x11Support);
  
negative.code: { stdenv, fetchurl, pkgconfig, freetype, yasm
  , fontconfigSupport ? true, fontconfig ? null, freefont_ttf ? null
  , fribidiSupport ? true, fribidi ? null
  , x11Support ? true, libX11 ? null, libXext ? null, mesa ? null
  , xineramaSupport ? true, libXinerama ? null
  , xvSupport ? true, libXv ? null
  