Python 3.11 loses the ability to set PYTHON_DECIMAL_WITH_MACHINE

Bug report

Python 3.10 had the ability to set PYTHON_DECIMAL_WITH_MACHINE to override the choice of configuration for the _decimal module:
https://github.com/python/cpython/blob/dcb342b5f9d931b030ca310bf3e175bbc54df5aa/setup.py#L2388-L2397

Since Python 3.11, this is no longer possible. This feature was necessary, on macOS particularly, with the --with-system-libmpdec option if that system libmpdec is configured differently to the default Python config. On macOS, the default Python config forces universal, while setting PYTHON_DECIMAL_WITH_MACHINE allowed it to be single-arch.

libmpdec produces different headers depending on how it was built, which is why the setting is important to be able to override. Without it, the _decimal module will fail to compile if the default does not match how system libmpdec was built.

Homebrew's Python currently depends on this feature.

A test within CPython also seems to depend on this feature: https://github.com/python/cpython/blob/f4c03484da59049eb62a9bf7777b963e2267d187/Modules/_decimal/tests/runall-memorydebugger.sh#L63-L79

Your environment

• CPython versions tested on: 3.11.0rc2
• Operating system and architecture: macOS 12 (x86_64 and arm64)

Compare the Python 3.10 setup.py lines linked in the report with the current build logic for _decimal, then inspect Modules/_decimal/tests/runall-memorydebugger.sh for its use of the setting. Confirm the override works with --with-system-libmpdec on the reported macOS architectures and that the relevant build and test checks pass.