building for older machines

[adamwolf]

this is a catch-all issue.

We have historically built on the oldest version we support, even though we have --macos-min-version. This is because of our brew dependencies that get included. If we don't --build-bottle, usually they are built for the current macos version.

If we do do --build-bottle, we're fine, except that the dependencies of our dependencies are not built with --build-bottle.

so

Add this information to the README.

Set up a 10.14 VM to install all the dependencies, and all their dependencies, with --build-bottle, until no dependencies are pulled in automatically.