[3.9] bpo-43568: Relax distutils MACOSX_DEPLOYMENT_TARGET check (GH-25827) (GH-26001)

[3.9] bpo-43568: Relax distutils MACOSX_DEPLOYMENT_TARGET check (GH-25827) (GH-26001)

Only complain if the config target is >= 10.3 and the current target is
< 10.3. The check was originally added to ensure that incompatible
LDSHARED flags are not used, because '-undefined dynamic_lookup' is
used when building for 10.3 and later, and is not supported on older OS
versions. Apart from that, there should be no problem in general
with using an older target. In particular, this allows targeting macOS
11.0 when Python was built for a newer minor version like 11.3.

(manually cherry picked from part of commit 8703178)