Properly use cmake variable in ONNXRuntime (#15776)

Co-authored-by: UltralyticsAssistant <web@ultralytics.com>

examples/YOLOv8-ONNXRuntime-CPP/CMakeLists.txt

@@ -49,6 +49,9 @@ elseif (APPLE)
     # set(ONNXRUNTIME_ROOT "${CMAKE_CURRENT_SOURCE_DIR}/onnxruntime-osx-x64-${ONNXRUNTIME_VERSION}")
     # Apple Universal binary
     # set(ONNXRUNTIME_ROOT "${CMAKE_CURRENT_SOURCE_DIR}/onnxruntime-osx-universal2-${ONNXRUNTIME_VERSION}")
+else ()
+    message(SEND_ERROR "Variable ONNXRUNTIME_ROOT is not set properly. Please check if your cmake project \
+    is not compiled with `-D WIN32=TRUE`, `-D LINUX=TRUE`, or `-D APPLE=TRUE`!")
 endif ()
 
 include_directories(${PROJECT_NAME} ${ONNXRUNTIME_ROOT}/include)

examples/YOLOv8-ONNXRuntime-CPP/README.md

@@ -82,6 +82,19 @@ Note (2): Due to ONNX Runtime, we need to use CUDA 11 and cuDNN 8. Keep in mind
    cmake ..
    ```
 
+   **Notice**:
+
+   If you encounter an error indicating that the `ONNXRUNTIME_ROOT` variable is not set correctly, you can resolve this by building the project using the appropriate command tailored to your system.
+
+   ```console
+   # compiled in a win32 system
+   cmake -D WIN32=TRUE ..
+   # compiled in a linux system
+   cmake -D LINUX=TRUE ..
+   # compiled in an apple system
+   cmake -D APPLE=TRUE ..
+   ```
+
 5. Build the project:
 
    ```console