anchor.diff: -{ stdenv, icu, expat, zlib, bzip2, python, fixDarwinDylibNames
  +{ stdenv, icu, expat, zlib, bzip2, python2
   , toolset ? if stdenv.cc.isClang then "clang" else null
   , enableRelease ? true
   , enableDebug ? false
  
positive.code:     EOF
    '';
  
    NIX_CFLAGS_LINK = stdenv.lib.optionalString stdenv.isDarwin
                        "-headerpad_max_install_names";
  
    enableParallelBuilding = true;
  
    buildInputs = [ icu expat zlib bzip2 python ]
      ++ stdenv.lib.optional stdenv.isDarwin fixDarwinDylibNames;
  
    configureScript = "./bootstrap.sh";
    configureFlags = commonConfigureFlags ++ [
      "--with-icu=${icu}"
      "--with-python=${python.interpreter}"
    ] ++ optional (toolset != null) "--with-toolset=${toolset}";
  
    buildPhase = builder nativeB2Args;
  
negative.code:     installPhase = installer crossB2Args;
      postFixup = fixup;
    };
  
    meta = {
      homepage = "http://boost.org/";
      description = "Collection of C++ libraries";
      license = stdenv.lib.licenses.boost;
      platforms = platforms.unix;
      maintainers = with maintainers; [ simons wkennington ];
    };
  }
  