error if cuda4dnn depends are not resolved

modules/dnn/CMakeLists.txt

@@ -27,8 +27,18 @@ ocv_option(OPENCV_DNN_CUDA "Build with CUDA support"
     AND HAVE_CUDNN
 )
 
-if(OPENCV_DNN_CUDA AND HAVE_CUDA AND HAVE_CUBLAS AND HAVE_CUDNN)
-  add_definitions(-DCV_CUDA4DNN=1)
+if(OPENCV_DNN_CUDA)
+  if(HAVE_CUDA AND HAVE_CUBLAS AND HAVE_CUDNN)
+    add_definitions(-DCV_CUDA4DNN=1)
+  else()
+    if(NOT HAVE_CUDA)
+      message(SEND_ERROR "DNN: CUDA backend requires CUDA Toolkit. Please resolve dependency or disable OPENCV_DNN_CUDA=OFF")
+    elseif(NOT HAVE_CUBLAS)
+      message(SEND_ERROR "DNN: CUDA backend requires cuBLAS. Please resolve dependency or disable OPENCV_DNN_CUDA=OFF")
+    elseif(NOT HAVE_CUDNN)
+      message(SEND_ERROR "DNN: CUDA backend requires cuDNN. Please resolve dependency or disable OPENCV_DNN_CUDA=OFF")
+    endif()
+  endif()
 endif()
 
 ocv_cmake_hook_append(INIT_MODULE_SOURCES_opencv_dnn "${CMAKE_CURRENT_LIST_DIR}/cmake/hooks/INIT_MODULE_SOURCES_opencv_dnn.cmake")