In case anyone else wants to play with Zig on webassembly, here's what you need to do to make it work on a mac today.
You'll need LLVM to output to the WASM target. This has just been added by default in trunk, so if LLVM >7 is available, you might be able to just brew install llvm
.
If you have wasm support already you should see:
$ llc --version