dnn: allow to use external protobuf

"custom layers" feature will not work properly in these builds.

modules/dnn/test/test_layers.cpp

@@ -1149,7 +1149,11 @@ private:
     int outWidth, outHeight, zoomFactor;
 };
 
+#ifndef OPENCV_DNN_EXTERNAL_PROTOBUF
 TEST_P(Test_Caffe_layers, Interp)
+#else
+TEST_P(Test_Caffe_layers, DISABLED_Interp)  // requires patched protobuf (available in OpenCV source tree only)
+#endif
 {
     if (backend == DNN_BACKEND_INFERENCE_ENGINE && target == DNN_TARGET_MYRIAD)
         throw SkipTestException("");