cmake: fix protobuf handling

modules/dnn/CMakeLists.txt

@@ -66,6 +66,9 @@ ocv_warnings_disable(CMAKE_CXX_FLAGS
     -Winvalid-offsetof                 # Apple Clang (attr_value.pb.cc)
 )
 
+set(include_dirs "")
+set(libs "")
+
 if(PROTOBUF_UPDATE_FILES)
   file(GLOB proto_files "${CMAKE_CURRENT_LIST_DIR}/src/tensorflow/*.proto" "${CMAKE_CURRENT_LIST_DIR}/src/caffe/opencv-caffe.proto" "${CMAKE_CURRENT_LIST_DIR}/src/onnx/opencv-onnx.proto")
   set(PROTOBUF_GENERATE_CPP_APPEND_PATH ON) # required for tensorflow
@@ -76,10 +79,15 @@ else()
   set(fw_inc "${CMAKE_CURRENT_LIST_DIR}/misc/caffe" "${CMAKE_CURRENT_LIST_DIR}/misc/tensorflow" "${CMAKE_CURRENT_LIST_DIR}/misc/onnx")
 endif()
 
-set(include_dirs ${fw_inc})
+list(APPEND include_dirs ${fw_inc})
+list(APPEND libs ${Protobuf_LIBRARIES})
+if(NOT BUILD_PROTOBUF)
+  list(APPEND include_dirs ${Protobuf_INCLUDE_DIRS})
+endif()
+
 set(sources_options "")
 
-set(libs libprotobuf ${LAPACK_LIBRARIES})
+list(APPEND libs ${LAPACK_LIBRARIES})
 if(OPENCV_DNN_OPENCL AND HAVE_OPENCL)
   list(APPEND include_dirs ${OPENCL_INCLUDE_DIRS})
 else()