Emscripten settings in configure.ac are not embedding friendly

Bug report

Bug description:

I would like to be able to opt out of certain Emscripten options since I am embedded CPython in a larger application. For example, the forced inclusion of the JS file system is breaking my builds since I use WASMFS in my system which isn't compatible with -lidbfs.js, -lnodefs.js, -lproxyfs.js, -lworkerfs.js. Moreover, I would like to customise the TOTAL_MEMORY option.

Perhaps these options can be on by default but placed behind a flag that can turn them off? For example, --enable-wasm-emscripten-jsfs? Alternatively, perhaps all these settings can be disabled unless one of node, browser, node-debug, browser-debug is set via --with_emscripten_target?

Right now, it seems my only option is to apply a patch to turn these settings off which should be unnecessary in my opinion. @hoodmane @vstinner what do you think?

CPython versions tested on:

CPython main branch

Operating systems tested on:

Other

Start by reading configure.ac around lines 2340-2351 and the existing --with_emscripten_target handling. Determine how the Emscripten JavaScript filesystem options and TOTAL_MEMORY are currently set, then define how embedded builds can opt out or customize them. Done means the requested settings can be controlled without maintaining a patch.