On a semi-fresh latest Intel Mac, here are the things I needed to do for ./mach bootstrap to succeed:

1. Make a pip executable under my local bin path, to get around the system only having pip3 now, and symlinking to pip3 won't work, either.

Here's the pass-through pip command:

#!/bin/bash
pip3 "$@"

Looks like mach needs to be updated to use pip3 on MacOS, instead of pip.

2. Install pkg-config.

I just needed brew install pkg-config

This is probably a missing dep from mach?

Anyways, hope this is helpful for something else trying to do the same, and maybe help patch up mach.