The build agents will now automatically install and activate the
Emscripten version found in minimum-versions.inc.sh when configuring
WASM projects. For developer machines, this behaviour can be enabled by
setting the environment variable `KEYMAN_USE_EMSDK` (recommended).
This will be back-ported to 17.0 so that we can ensure that all builds
use a consistent Emscripten version.
`emcc.py` is not marked as executable in emcripten's git repo so the
build failed when trying to locate emscripten. However, it turns out
that `emcc` is marked as executable, so we use that instead.
On Windows however, we still need to use `emcc.py` because otherwise
Meson won't detect it as valid compiler.
- change the common emscripten locator to warn if $EMCC or $EMSCRIPTEN_BASE/emcc.py is present but not executable
- emscripten v3.1.50 via homebrew seems to be not executable
