added cuda cmake flags (allows GPU acceleration of some of the libraries functions)

pkgs/development/libraries/dlib/default.nix

@@ -3,6 +3,7 @@
 
   # see http://dlib.net/compile.html
 , avxSupport ? true
+, cudaSupport ? true
 }:
 
 stdenv.mkDerivation rec {
@@ -20,7 +21,9 @@ stdenv.mkDerivation rec {
     rm -rf dlib/external
   '';
 
-  cmakeFlags = [ "-DUSE_AVX_INSTRUCTIONS=${if avxSupport then "yes" else "no"}" ];
+  cmakeFlags = [ 
+    "-DUSE_DLIB_USE_CUDA=${if cudaSupport then "1" else "0"}"
+    "-DUSE_AVX_INSTRUCTIONS=${if avxSupport then "yes" else "no"}" ];
 
   enableParallelBuilding = true;
   nativeBuildInputs = [ cmake pkgconfig ];