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:       ${if pulseSupport then "--enable-pulse" else "--disable-pulse"}
        ${optionalString (useUnfreeCodecs && codecs != null) "--codecsdir=${codecs}"}
        ${optionalString (stdenv.isi686 || stdenv.isx86_64) "--enable-runtime-cpudetection"}
        ${optionalString fribidiSupport "--enable-fribidi"}
        --disable-xanim
        --disable-ivtv
        --disable-xvid --disable-xvid-lavc
  