fix(setup): sync with cmake_example

[henryiii]

The current version of the setup.py wasn't loading the correct MSVC reliably. I've synced this up with the work done on the cmake_example, which fixes this issue and is arguably a bit nicer with more docs.

[henryiii]

FYI, the current pybind/cmake_example was heavily influenced by awkward's setup.py, so it's coming full circle. :)

[jpivarski]

This looks good to me: adding more CMake environment variables and a mapping of platform names, presumably to build the more fine-grained matrix of wheels. The only thing that sticks out to me is a mixture of += and append for accumulating arguments, but that's a minor stylistic thing. I'm relying heavily on the fact that the tests pass and that you definitely know what you're doing.

So I'd merge this as-is, and since your last commit was 19 hours ago (and you just asked me to review), I assume you're done and I'll merge it right away.

[henryiii]

The only thing that sticks out to me is a mixture of += and append

I had intended to make all the one-item additions an append, but got missed. Might try to clean that up later, then.