cmake: revise OPENCV_DNN_BACKEND_DEFAULT integration

- disable message on default value

CMakeLists.txt

@@ -1672,7 +1672,7 @@ else()
   endif()
 endif()
 
-if(BUILD_opencv_dnn)
+if(BUILD_opencv_dnn AND OPENCV_DNN_BACKEND_DEFAULT)
     status("    Default DNN backend:" ${OPENCV_DNN_BACKEND_DEFAULT})
 endif()
 

modules/dnn/CMakeLists.txt

@@ -227,9 +227,10 @@ if(TARGET ocv.3rdparty.openvino AND OPENCV_DNN_OPENVINO)
   endif()
 endif()
 
-set(OPENCV_DNN_BACKEND_DEFAULT "DNN_BACKEND_OPENCV" CACHE STRING "Default backend used by the DNN module")
-ocv_append_source_file_compile_definitions("${CMAKE_CURRENT_LIST_DIR}/src/dnn_params.cpp" "OPENCV_DNN_BACKEND_DEFAULT=${OPENCV_DNN_BACKEND_DEFAULT}")
-
+set(OPENCV_DNN_BACKEND_DEFAULT "" CACHE STRING "Default backend used by the DNN module (DNN_BACKEND_OPENCV if empty)")
+if(OPENCV_DNN_BACKEND_DEFAULT)
+  ocv_append_source_file_compile_definitions("${CMAKE_CURRENT_LIST_DIR}/src/dnn_params.cpp" "OPENCV_DNN_BACKEND_DEFAULT=${OPENCV_DNN_BACKEND_DEFAULT}")
+endif()
 
 ocv_install_used_external_targets(${libs} ${dnn_runtime_libs})
 

modules/dnn/src/dnn_params.cpp

@@ -36,7 +36,11 @@ bool getParam_DNN_OPENCL_ALLOW_ALL_DEVICES()
 int getParam_DNN_BACKEND_DEFAULT()
 {
     static int PARAM_DNN_BACKEND_DEFAULT = (int)utils::getConfigurationParameterSizeT("OPENCV_DNN_BACKEND_DEFAULT",
+#ifdef OPENCV_DNN_BACKEND_DEFAULT
             (size_t)OPENCV_DNN_BACKEND_DEFAULT
+#else
+            (size_t)DNN_BACKEND_OPENCV
+#endif
     );
     return PARAM_DNN_BACKEND_DEFAULT;
 }