m4: fix Python 3.13/3.14 detection and empty version string crash

[blshkv]

Summary

• Add local m4/ax_python.m4 with python3.13/python3.14 added to the binary search list, and distutils replaced with sysconfig
• Add local m4/ax_python_devel.m4 with a guard in vtup() against an empty version string
• Add local m4/ax_boost_python.m4 so AX_BOOST_PYTHON is available to aclocal

Details

Fix 1: Python 3.13/3.14 not detected (ax_python.m4)

The for python in ... loop stopped at python3.12, so Python 3.13+ was never found. PYTHON_INCLUDE_DIR stayed empty, causing:

fatal error: pyconfig.h: No such file or directory

Fix 2: distutils removed in Python 3.12+ (ax_python.m4)

The header path was obtained via:

from distutils.sysconfig import *; print(get_config_var('CONFINCLUDEPY'))

distutils was removed in Python 3.12. Replaced with:

import sysconfig; print(sysconfig.get_config_var('CONFINCLUDEPY') or sysconfig.get_path('include'))

Fix 3: vtup() crashes on empty version string (ax_python_devel.m4)

AX_PYTHON_DEVEL is called with >= '$PYTHON_VERSION'. If ax_python_bin is plain python (no version suffix), cut -c7- produces an empty string. vtup('') then calls int('') which raises ValueError.

 def vtup(self, s):
+    if not s.strip(): return ()
     return tuple(map(int, s.strip().replace("rc", ".").split(".")))

Fix 4: AX_BOOST_PYTHON not found (ax_boost_python.m4)

Without a local copy of ax_boost_python.m4 in m4/, aclocal fails to include it and configure exits with:

./configure: line NNNNN: AX_BOOST_PYTHON: command not found

Closes #25

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