Merge pull request #14660 from YashasSamaga:dnn-cuda-build

add cuDNN dependency and setup build for cuda4dnn (#14660)

* update cmake for cuda4dnn

- Adds FindCUDNN
- Adds new options:
   * WITH_CUDA
   * OPENCV_DNN_CUDA
- Adds CUDA4DNN preprocessor symbol for the DNN module

* FIX: append EXCLUDE_CUDA instead of overwrite

* remove cuDNN dependency for user apps

* fix unused variable warning

modules/dnn/src/precomp.hpp

@@ -53,10 +53,22 @@
 #else
 #undef HAVE_OPENCL
 #endif
+
+#ifndef CV_CUDA4DNN
+#define CV_CUDA4DNN 0
+#endif
+
+#if CV_CUDA4DNN
+#ifndef HAVE_CUDA
+#error "Configuration error: re-run CMake from clean build directory"
+#endif
+#else
+#undef HAVE_CUDA
+#endif
+
 #include <opencv2/core/ocl.hpp>
 #include <opencv2/core/opencl/ocl_defs.hpp>
 
-
 #include <opencv2/core/utils/trace.hpp>
 #include <opencv2/dnn.hpp>
 #include <opencv2/dnn/all_layers.hpp>