BLD: for C extension builds on mac, target macOS 10.9 where possible
- [x] closes #23424
- [x] tests added / passed 
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

For extension builds, `distutils` targets the same macOS version as python was built for, even if the host system is newer. This can cause problems when building on a 10.9+ system, with a python built for <10.9. Targetting macOS <10.9 sets the c++ standard library to `stdlibc++`, which was deprecated in 10.9 in favour of `libc++`. Starting with Xcode 10, `stdlibc++` is no longer installed by default, so isn't guaranteed to be available. This can be an issue with the current 64/32b universal builds of python, which target macOS 10.7.

Another reason to do this is to move over to 10.9 / `libc++` where possible, as the current 64-bit builds of python have done.