Namespace Autoconf preprocessor defines

[erlend-aasland]

Quoting from capi-workgroup/problems#46:

The headers define a number of identifiers that don't start with Py (or _Py), potentially clashing with other code.

[...]

• configure cruft like HAVE_STRFTIM or SIZEOF_SIZE_T

We can add an m4 macro that wraps AC_DEFINE and AH_TEMPLATE, to make sure we namespace any defines produced by configure. I'm not sure how PEP-387 plays in here; can we simply change these preprocessor defines? I guess not, since what's exposed in Python.h is protected by PEP-387. For now, let's just add macros that add properly namespaced defined in addition to the existing ones.

• Add macros that add Py_ namespaced defined in addition to the existing defs
  • Add PY_DEFINE macro that wraps AC_DEFINE
  • Add PY_TEMPLATE macro that wraps AH_TEMPLATE
  • Add PY_CHECK_HEADERS macro that wraps AC_CHECK_HEADERS
  • Use these macros in configure.ac
• Remove the old non-namespaced defs

Linked PRs

• gh-105813: Proof-of-concept: Add PY_DEFINE macro that wraps AC_DEFINE #115343
• gh-105813: Autoconf: add and use PY_DEFINE macro #128525