_ctypes fails to import on NetBSD due to a missing `alloca` symbol

[collinfunk]

Bug report

Bug description:

After running ./configure && gmake on a NetBSD 10 machine, the build finishes with the following warning:

Written build/lib.netbsd-10.0-evbarm-3.15/_sysconfigdata__netbsd10_.py
Written build/lib.netbsd-10.0-evbarm-3.15/_sysconfig_vars__netbsd10_.json
./python -E -c 'import sys ; from sysconfig import get_platform ; print("%s-%d.%d" % (get_platform(), *sys.version_info[:2]))' >platform
./python -E ./Tools/build/generate-build-details.py `cat pybuilddir.txt`/build-details.json
[ERROR] _ctypes failed to import: /home/collinfunk/cpython/build/lib.netbsd-10.0-evbarm-3.15/_ctypes.cpython-315.so: Undefined PLT symbol "alloca" (symnum = 48)

This is due to an incomplete check for alloca in the _ctypes.h module.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Other

Linked PRs

• gh-134486: Fix missing alloca symbol in _ctypes on NetBSD. #134487