bpo-41129: Fix check for macOS SDK paths when building Python (GH-25785)

Narrow search to match contents of SDKs, namely only files in ``/System/Library``,
``/System/IOSSupport``, and ``/usr`` other than ``/usr/local``. Previously,
anything under ``/System`` was assumed to be in an SDK which causes problems
with the new file system layout in 10.15+ where user file systems may appear
to be mounted under ``/System``.  Paths in ``/Library`` were also
incorrectly treated as SDK locations.

Co-authored-by: Ned Deily <nad@python.org>