bpo-38360: macOS: support alternate form of -isysroot flag (GH-16480)

It is possible to use either '-isysroot /some/path' (with a space) or
'-isysroot/some/path' (no space in between). Support both forms in
places where special handling of -isysroot is done, rather than just
the first form.
Co-authored-by: Ned Deily <nad@python.org>