gh-94404: makesetup: use correct CFLAGS and macOS workaround (GH-94405)

``makesetup`` now works around an issue with sed on macOS and uses correct
CFLAGS for object files that end up in a shared extension.
(cherry picked from commit 5150cbcd6821c0cf79b81cfc8780087bbc6985da)

Co-authored-by: Christian Heimes <christian@python.org>