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/libraries/dlib/default.nix

@@ -1,5 +1,8 @@
 { stdenv, lib, fetchFromGitHub, cmake, pkgconfig, libpng, libjpeg
 , guiSupport ? false, libX11
+
+  # see http://dlib.net/compile.html
+, avxSupport ? true
 }:
 
 stdenv.mkDerivation rec {
@@ -17,6 +20,8 @@ stdenv.mkDerivation rec {
     rm -rf dlib/external
   '';
 
+  cmakeFlags = [ "-DUSE_AVX_INSTRUCTIONS=${if avxSupport then "yes" else "no"}" ];
+
   enableParallelBuilding = true;
   nativeBuildInputs = [ cmake pkgconfig ];
   buildInputs = [ libpng libjpeg ] ++ lib.optional guiSupport libX11;