Cannot override SDL2_net with EMCC_LOCAL_PORTS

If I set the `EMCC_LOCAL_PORTS` environment variable to `sdl2_net=/path/to/dir`, I see the following error:

```
system_libs:ERROR: port sdl2_net lacks .SUBDIR attribute, which we need in order to override it locally, please update it
```

Is this because `SUBDIR` is missing in `tools/ports/sdl2_net.py`?

What's necessary to permit the use of `EMCC_LOCAL_PORTS` for a given library?