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

[hoodmane]

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

emscripten/emcc.py

Line 1565 in ebe0ddd

diagnostics.warning('emcc', 'linking a library with `-shared` will emit a static object file. This is a form of emulation to support existing build systems. If you want to build a runtime shared library use the SIDE_MODULE setting.')

to set settings.SIDE_MODULE = True but it would be helpful to have a feature for this.

[hoodmane]

The comment there says:

 # Until we have a better story for actually producing runtime shared libraries
 # we support a compatibility mode where shared libraries are actually just
 # object files linked with `wasm-ld --relocatable` or `llvm-link` in the case
 # of LTO.

But Emscripten has a good system for producing shared libraries =)

[sbc100]

Perhaps related: #16281