The `-undefined dynamic_lookup` option can only be used in LDSHARED on Mac OS X 10.3 and later. There is a fallback to explicitly linking with the framework for 10.2 and earlier. I'm pretty sure that currently supported Python versions don't build on 10.2 or older for several other reasons (I happen to know that even building on 10.5 requires a little patching.) So it's probably reasonable to just drop this code path.

There is a closely related check in distutils, though you would only know it's related if you looked through the history as I did. It errors out if you try to build a module for an older MACOSX_DEPLOYMENT_TARGET than Python was configured with. The purpose of that is to prevent using the wrong LDSHARED flags for the target platform. If 10.2 support is dropped, that check can be removed entirely.

I am aware that distutils is deprecated, going away, etc., and I am submitting a PR to setuptools as well. But setuptools does not yet override the stdlib distutils with its own by default, so bugs in the stdlib copy are still relevant.

If it's decided to keep 10.2 support, the check in distutils should still be relaxed to error only if the current MDT is < 10.3 and the configured MDT is >= 10.3. I can easily put together a PR for that if needed.

Either way, the approach taken in setuptools will depend on how LDSHARED is handled here.