[bug#54412] [PATCH] python: use the posix_prefix sysconfig scheme on Debian

Fixes Debian's bug: https://bugs.debian.org/1006784

Debian adds a custom sysconfig scheme to system python installs,
"posix_local". This is the default scheme, and it redirects local users'
Python module installs to /usr/local even though Python is installed
with a /usr prefix. Both are on Debian's python's sys.path module search
path.

Automake and its users understand prefixes, and are likely to select
/usr/local, explicitly. Select the "posix_prefix" scheme, with the
user-supplied prefix.

Previously this custom sysconfig scheme was specified in
distutils.sysconfig, but not sysconfig itself. As distutils is being
deprecated, the custom scheme is now specified in sysconfig, since
Debian's Python 3.10 (3.10.2-4).

Thank you, Gianfranco Costamagna <[email protected]>, for porting
the patch to automake.

* m4/python.m4: use the posix_prefix sysconfig scheme on Debian

Copyright-paperwork-exempt: yes
---
 m4/python.m4 | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/m4/python.m4 b/m4/python.m4
index 6653e4d89..51e050c19 100644
--- a/m4/python.m4
+++ b/m4/python.m4
@@ -255,7 +255,11 @@ except ImportError:
    am_cv_python_pythondir=`$PYTHON -c "
 $am_python_setup_sysconfig
 if can_use_sysconfig:
-  sitedir = sysconfig.get_path('purelib', vars={'base':'$am_py_prefix'})
+  scheme = sysconfig.get_default_scheme()
+  if scheme == 'posix_local':
+    # Debian's default scheme installs to /usr/local/ but we want to find headers in /usr/
+    scheme = 'posix_prefix'
+  sitedir = sysconfig.get_path('purelib', scheme, vars={'base':'$am_py_prefix'})
 else:
   from distutils import sysconfig
   sitedir = sysconfig.get_python_lib(0, 0, prefix='$am_py_prefix')
@@ -297,7 +301,11 @@ sys.stdout.write(sitedir)"`
    am_cv_python_pyexecdir=`$PYTHON -c "
 $am_python_setup_sysconfig
 if can_use_sysconfig:
-  sitedir = sysconfig.get_path('platlib', vars={'platbase':'$am_py_exec_prefix'})
+  scheme = sysconfig.get_default_scheme()
+  if scheme == 'posix_local':
+    # Debian's default scheme installs to /usr/local/ but we want to find headers in /usr/
+    scheme = 'posix_prefix'
+  sitedir = sysconfig.get_path('platlib', scheme, vars={'platbase':'$am_py_exec_prefix'})
 else:
   from distutils import sysconfig
   sitedir = sysconfig.get_python_lib(1, 0, prefix='$am_py_exec_prefix')
-- 
2.35.1