[InterNetNews/inn] 3c3483: Request posix_prefix paths from Python sysconfig

[Russ Allbery]

  Branch: refs/heads/main
  Home:   https://github.com/InterNetNews/inn
  Commit: 3c3483004a50645b122ea40ff690b2cedf4d3845
      https://github.com/InterNetNews/inn/commit/3c3483004a50645b122ea40ff690b2cedf4d3845
  Author: Russ Allbery <eagle at eyrie.org>
  Date:   2021-10-16 (Sat, 16 Oct 2021)

  Changed paths:
    M m4/python.m4

  Log Message:
  -----------
  Request posix_prefix paths from Python sysconfig

When using the Python sysconfig module to determine paths,
explicitly request the posix_prefix scheme.  This is the default
in current versions of Python, but in Python 2.7 as packaged in
Debian buster, the default scheme is posix_local.  This breaks
determination of the include path for Python.h, since it instead
returns the include path used for locally-installed modules.

Explicitly requesting the posix_prefix scheme avoids this problem.