How to deactivate

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

I tried the method [](https://stackoverflow.com/questions/15053776/how-do-you-disable-the-unused-variable-warnings-coming-out-of-gcc-in-3rd-party-c), namely, add `-Wno-unused-variable` in my **CMAKE_CXX_FLAGS**, however, I still obtained **ERRORs** as following:

```console
......
....../Open3D/cpp/open3d/io/file_format/FileGLTF.cpp: In function ‘bool open3d::io::ReadTriangleMeshFromGLTF(const string&, open3d::geometry::TriangleMesh&, const open3d::io::ReadTriangleMeshOptions&)’:
....../Open3D/cpp/open3d/io/file_format/FileGLTF.cpp:96:62: error: unused parameter ‘params’ [-Werror=unused-parameter]
96 | const ReadTriangleMeshOptions& params /*={}*/) {
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~
....../Open3D/cpp/open3d/io/file_format/FileGLTF.cpp: In function ‘bool open3d::io::WriteTriangleMeshToGLTF(const string&, const open3d::geometry::TriangleMesh&, bool, bool, bool, bool, bool, bool)’:
....../Open3D/cpp/open3d/io/file_format/FileGLTF.cpp:393:35: error: unused parameter ‘write_ascii’ [-Werror=unused-parameter]
393 | bool write_ascii /* = false*/,
| ~~~~~^~~~~~~~~~~
....../Open3D/cpp/open3d/io/file_format/FileGLTF.cpp:394:35: error: unused parameter ‘compressed’ [-Werror=unused-parameter]
394 | bool compressed /* = false*/,
| ~~~~~^~~~~~~~~~
....../Open3D/cpp/open3d/io/file_format/FileGLTF.cpp:398:35: error: unused parameter ‘print_progress’ [-Werror=unused-parameter]
398 | bool print_progress) {
| ~~~~~^~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
make[2]: *** [cpp/open3d/io/CMakeFiles/io.dir/build.make:289: cpp/open3d/io/CMakeFiles/io.dir/file_format/FileGLTF.cpp.o] Error 1
......
....../Open3D/cpp/open3d/t/io/file_format/FileXYZI.cpp:41:70: error: unused parameter ‘path’ [-Werror=unused-parameter]
41 | open3d::io::FileGeometry ReadFileGeometryTypeXYZI(const std::string &path) {
```