@freakboy3742's desired improvements to the Emscripten build script:

1. clean should be a little more selective. It currently destroys all of cross-build; it should only be purging the parts of cross-build that are emscripten specific (so that Emscripten, iOS and Android builds can co-exist). This is admittedly a "me" problem... but it's a problem I have :-)
2. The cross-build directory name should be configurable. Again, this is somewhat a "me" problem... but I currently need to have cross builds for 3.10-3.15, and they can't currently co-exist.
3. We shouldn't be building dependencies (libFFI and mpdec) on every build. A similar caching strategy to the one described for EMSDK should be possible - look to this directory; if the cache has a build for the right emscripten version, use it rather than rebuilding.
4. Add install-emscripten subcommand

Linked PRs

• gh-145219: Emscripten bld configurable directory & only clean Emscripten #145581
• [3.14] gh-145219: Add Emscripten cross-build and clean configurability (GH-145581) #145654
• gh-145219: Cache Emscripten libffi and mpdec builds, add install-emscripten cmd #145664
• [3.14] gh-145219: Cache Emscripten build dependencies, add install-emscripten (GH-145664) #145790