anchor.diff: -{ stdenv, icu, expat, zlib, bzip2, python, fixDarwinDylibNames
  +{ stdenv, fetchurl, icu, expat, zlib, bzip2, python, fixDarwinDylibNames
   , toolset ? if stdenv.cc.isClang then "clang" else null
   , enableRelease ? true
   , enableDebug ? false
  
positive.code:     buildPhase = builder crossB2Args;
      installPhase = installer crossB2Args;
      postFixup = fixup;
    };
  }
  
negative.code:   ] ++ optionals stdenv.isCrossWin [
      "target-os=windows"
      "threadapi=win32"
      "binary-format=pe"
      "address-model=${if stdenv.isCross64 then "64" else "32"}"
      "architecture=x86"
    ];
    crossB2Args = concatStringsSep " " (genericB2Flags ++ crossB2Flags);
  
  