anchor.diff:  
   assert elem precision [ "single" "double" "long-double" "quad-precision" ];
   
  -let version = "3.3.6-pl1"; in
  +let
  +  version = "3.3.6-pl1";
  +  withDoc = stdenv.cc.isGNU;
  +in
   
   stdenv.mkDerivation rec {
     name = "fftw-${precision}-${version}";
  
positive.code:     ++ optional (stdenv.isx86_64 && (precision == "single" || precision == "double") )  "--enable-sse2"
      ++ optional stdenv.cc.isGNU "--enable-openmp"
      # doc generation causes Fortran wrapper generation which hard-codes gcc
      ++ optional (!stdenv.cc.isGNU) "--disable-doc";
  
    enableParallelBuilding = true;
  
  
negative.code:     sha256 = "0g8qk98lgq770ixdf7n36yd5xjsgm2v3wzvnphwmhy6r4y2amx0y";
    };
  
    outputs = [ "out" "dev" "man" ]
      ++ optional withDoc "info"; # it's dev-doc only
    outputBin = "dev"; # fftw-wisdom
  
    configureFlags =
  