Feature request: environment variable to respect the `-shared` flag

In Pyodide, we run into a fair amount of trouble convincing various build systems to pass `-s SIDE_MODULE=1` through to the compiler. Also, if a library asks to be `-shared` we always want to respect it. I finally thought to just patch
https://github.com/emscripten-core/emscripten/blob/ebe0ddde03145c0a2a197961bdf70dfde35cf957/emcc.py#L1565
to set `settings.SIDE_MODULE = True` but it would be helpful to have a feature for this.