anchor.diff:  { lib, stdenv
  -, fetchurl, fetchFromGitHub
  +, fetchurl, fetchFromGitHub, fetchpatch
   , cmake, pkgconfig, unzip, zlib, pcre, hdf5
   , caffe, glog, boost, google-gflags, protobuf
   , config
  
positive.code: stdenv.mkDerivation rec {
    name = "opencv-${version}";
    inherit version src;
  
    postUnpack = lib.optionalString buildContrib ''
      cp --no-preserve=mode -r "${contribSrc}/modules" "$NIX_BUILD_TOP/opencv_contrib"
  
negative.code: 
    enableParallelBuilding = true;
  
    postBuild = lib.optionalString enableDocs ''
      make doxygen
    '';
  