anchor.diff:  { lib, stdenv, fetchurl, pkg-config, yasm
   , freetype, fribidi, harfbuzz
  -, encaSupport ? true, enca ? null # enca support
   , fontconfigSupport ? true, fontconfig ? null # fontconfig support
   , rasterizerSupport ? false # Internal rasterizer
   , largeTilesSupport ? false # Use larger tiles in the rasterizer
   , libiconv
   }:
   
  -assert encaSupport -> enca != null;
   assert fontconfigSupport -> fontconfig != null;
   
   let
  
positive.code:   };
  
    configureFlags = [
      (mkFlag encaSupport "enca")
      (mkFlag fontconfigSupport "fontconfig")
      (mkFlag rasterizerSupport "rasterizer")
      (mkFlag largeTilesSupport "large-tiles")
  
negative.code:   nativeBuildInputs = [ pkg-config yasm ];
  
    buildInputs = [ freetype fribidi harfbuzz ]
      ++ optional fontconfigSupport fontconfig
      ++ optional stdenv.isDarwin libiconv;
  
  