anchor.diff:  assert postprocVisualizerSupport -> postprocSupport;
   assert unitTestsSupport -> curl != null && coreutils != null;
   assert vp9HighbitdepthSupport -> (vp9DecoderSupport || vp9EncoderSupport);
  -assert isCygwin -> unitTestsSupport && webmIOSupport && libyuvSupport;
   
   stdenv.mkDerivation rec {
     name = "libvpx-${version}";
  
positive.code:     (enableFeature gprofSupport "gprof")
      (enableFeature gcovSupport "gcov")
      # Required to build shared libraries
      (enableFeature (!isCygwin) "pic")
      (enableFeature (isi686 || isx86_64) "use-x86inc")
      (enableFeature optimizationsSupport "optimizations")
      (enableFeature runtimeCpuDetectSupport "runtime-cpu-detect")
  
negative.code:     (enableFeature ontheflyBitpackingSupport "onthefly-bitpacking")
      (enableFeature errorConcealmentSupport "error-concealment")
      # Shared libraries are only supported on ELF platforms
      "--disable-static --enable-shared"
      (enableFeature smallSupport "small")
      (enableFeature postprocVisualizerSupport "postproc-visualizer")
      (enableFeature unitTestsSupport "unit-tests")
  