gh-105813: Autoconf: add and use PY_DEFINE macro

[erlend-aasland]

• Issue: Namespace Autoconf preprocessor defines #105813

[erlend-aasland]

Finally got around to look at this again. I think this may be a good start, @encukou. Re. capi-workgroup/problems#46. I excluded defines like _NETBSD_SOURCE and similar. Perhaps we should limit this PR to only HAVE_ and WITH_ defines for now.

[encukou]

That does look like a great start! Not familiar enough with Autotools to comment on the implementation.
But, it someone who's not an Autotools expert can now experiment solving the other issues (naming scheme and migration path).

I don't think limiting this to Py_HAVE_ and Py_WITH is worth it. We might want to change the prefix anyway (perhaps to _PY_ -- https://discuss.python.org/t/py-prefix-for-if-friendly-macros/60825 and private so we can remove without deprecation).
Perhaps the ones other than Py_HAVE_ & Py_WITH_ need another common prefix (PyPlatform_?)... but if they end up private, probably not.

One thing I'm worried about is keeping the two sets of names in sync. I'm not sure if/how a redistributor can set something like POSIX_SEMAPHORES_NOT_ENABLED, and how that is propagated to the Py_-prefixed version.

[picnixz]

Out of curiosity, do you want to allow defining private macros (namely _Py) or not?

[picnixz]

If you change the Py macro for whatever reason, the non-prefixed Py_ macro would not be changed. Instead, it should be aliased to [Py_]#1 directly.