bpo-41116: Ensure system supplied libraries are found on macOS 11 (GH-23301) (GH-23455)

On macOS system provided libraries are in a shared library cache
and not at their usual location. This PR teaches distutils to search
in the SDK, even if there was no "-sysroot" argument in
the compiler flags.
(cherry picked from commit 404a719b5127602c1a948f8e189ab61cd3f147d8)

Co-authored-by: Ronald Oussoren <ronaldoussoren@mac.com>
4 files changed