anchor.diff:  , enablePIC ? false
   , enableExceptions ? false
   , taggedLayout ? ((enableRelease && enableDebug) || (enableSingleThreaded && enableMultiThreaded) || (enableShared && enableStatic))
  +, mpi ? null
   
   # Attributes inherit from specific versions
   , version, src
  
positive.code:   nativeB2Flags = [
      "-sEXPAT_INCLUDE=${expat}/include"
      "-sEXPAT_LIBPATH=${expat}/lib"
    ] ++ optional (toolset != null) "toolset=${toolset}";
    nativeB2Args = concatStringsSep " " (genericB2Flags ++ nativeB2Flags);
  
    crossB2Flags = [
  
negative.code:         substituteInPlace tools/build/src/tools/clang-darwin.jam \
            --replace '$(<[1]:D=)' "$lib/lib/\$(<[1]:D=)";
      fi;
    '' + optionalString (mpi != null) ''
      cat << EOF > user-config.jam
      using mpi : ${mpi}/bin/mpiCC ;
      EOF
    '';
  
    NIX_CFLAGS_LINK = stdenv.lib.optionalString stdenv.isDarwin
  