dlib: add flag to disable AVX support

Especially older hardware doesn't support AVX instructions. DLib is
still functional there, but significantly slower[1].

By setting `avxInstructions` to false, DLib will be compiled without
this feature.

[1] http://dlib.net/compile.html

pkgs/development/python-modules/dlib/default.nix

@@ -1,12 +1,17 @@
-{ buildPythonPackage, dlib, python, pytest }:
+{ buildPythonPackage, dlib, python, pytest, avxSupport ? true }:
 
 buildPythonPackage {
   inherit (dlib) name src nativeBuildInputs buildInputs meta;
 
+  # although AVX can be enabled, we never test with it. Some Hydra machines
+  # fail because of this, however their build results are probably used on hardware
+  # with AVX support.
   checkPhase = ''
     ${python.interpreter} nix_run_setup test --no USE_AVX_INSTRUCTIONS
   '';
 
+  setupPyBuildFlags = [ "--${if avxSupport then "yes" else "no"} USE_AVX_INSTRUCTIONS" ];
+
   patches = [ ./build-cores.patch ];
 
   checkInputs = [ pytest ];