fix: follow rest of pybind11 closer with PYBIND11_HAS_SUBINTERPRETER_SUPPORT

[henryiii]

Description

See #5708, this is closer to how the other HAS_ defines work, while still supporting a user override.

Suggested changelog entry:

• Internal adjustment to PYBIND11_HAS_SUBINTERPRETER_SUPPORT.

[Copilot]

Pull Request Overview

This PR adjusts the handling of subinterpreter support to more closely mirror other PYBIND11 configuration defines and to allow for user override. Key changes include:

• Updating test files (both Python and C++) to reference the new attribute "defined_PYBIND11_HAS_SUBINTERPRETER_SUPPORT".
• Modifying preprocessor checks in header files to use "#ifdef"/"#ifndef" and adjusting macro logic in common.h.
• Ensuring consistency between shared and per interpreter modules in how subinterpreter support is indicated.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/test_multiple_interpreters.py	Updated attribute check from PYBIND11_HAS_SUBINTERPRETER_SUPPORT to defined_PYBIND11_HAS_SUBINTERPRETER_SUPPORT.
tests/test_embed/test_subinterpreter.cpp	Switched from "#if" to "#ifdef" for subinterpreter support macro conditional.
tests/mod_shared_interpreter_gil.cpp	Revised module attribute assignment to reflect the new user override approach.
tests/mod_per_interpreter_gil.cpp	Revised module attribute assignment to reflect the new user override approach.
include/pybind11/subinterpreter.h	Changed macro guard from "#if !PYBIND11_HAS_SUBINTERPRETER_SUPPORT" to "#ifndef" for clarity.
include/pybind11/detail/internals.h	Updated macro check from "#if" to "#ifdef" to align with the revised handling.
include/pybind11/detail/common.h	Added user override logic and updated the macro definition/undefining conditions.

Comments suppressed due to low confidence (1)

tests/test_multiple_interpreters.py:30

• The updated attribute name 'defined_PYBIND11_HAS_SUBINTERPRETER_SUPPORT' is clear given the user override changes; consider updating the documentation to clarify its intentional distinction from 'PYBIND11_HAS_SUBINTERPRETER_SUPPORT'.

if not m.defined_PYBIND11_HAS_SUBINTERPRETER_SUPPORT:

[rwgk]

Thanks @henryiii!