I don't think WASM=1 in the env has an effect, but maybe I forgot something (is it mentioned in the docs somewhere?). The warning is saying that that library is not needed when using the asm.js backend.
That env var means it tries to use the wasm backend, but the wasm backend is not built with emscripten's default LLVM (since it isn't stable yet). Instead you'd need to build LLVM (with that backend enabled, I think they don't have it enabled by default, it's experimental), and point emscripten to use that (e.g. by setting the LLVM env var).
It should say after it builds where it placed the outputs, they will be in the cache dir, usually something like
I'm not sure, might want to ask in that PR.