How to Build

You need to install Emscripten from https://emscripten.org/docs/getting_started/downloads.html, and have the environment variables set, as described in https://emscripten.org/docs/getting_started/downloads.html#installation-instructions
Depending on your configuration, in Windows you may need to run emsdk/emsdk_env.bat in your console to access the Emscripten command-line tools.
Then build using make while in the example_emscripten/ directory.
For local testing, you may need a local webserver. Quoting https://emscripten.org/docs/getting_started:
"Unfortunately several browsers (including Chrome, Safari, and Internet Explorer) do not support file:// XHR requests, and can’t load extra files needed by the HTML (like a .wasm file, or packaged file data as mentioned lower down). For these browsers you’ll need to serve the files using a local webserver and then open http://localhost:8000/hello.html."
Easy local webserver: "For example, Python has one built in, python -m http.server in Python 3 or python -m SimpleHTTPServer in Python 2. After doing that, you can visit http://localhost:8000/."

Obsolete features:

Emscripten 2.0 (August 2020) obsoleted the fastcomp back-end, only llvm is supported.
Emscripten 1.39.0 (October 2019) obsoleted the BINARYEN_TRAP_MODE=clamp compilation flag which was required with version older than 1.39.0 to avoid rendering artefacts. See #2877 for details. If you use an older version, uncomment this line in the Makefile:

#EMS += -s BINARYEN_TRAP_MODE=clamp