Use macOS sandbox to isolate system dependencies

#545 was because we were setting CC but not CXX, meaning we were using our own cc command but /usr/bin/c++.

I think we can use the macOS built-in sandboxing system (see `man sandbox-exec` and `man sandbox_init`, see also /System/Library/Sandbox/Profiles/ for examples of the syntax) to restrict access to /usr/bin/c++ and friends, so the build fails if you attempt to use it.

... honestly this kind of sounds like https://github.com/twosigma/ts_isolate, I wonder if we should use that on Linux and whether I should add a macOS implementation with the same API, backed by `sandbox_init`.