PEP 831: clarify build backend flag propagation

[cc0006-coder]

This clarifies how PEP 831's compiler flags reach extension builds:

• It identifies Setuptools and other build systems that deliberately consume sysconfig as the consumers that inherit the flags automatically.
• It states that other PEP 517 backends do not inherit the flags unless they explicitly support them.
• It documents build-details.json (PEP 739) as the future standardized interface for exposing these build details to backends.

The change is limited to the existing propagation discussion and does not alter the proposal itself.

Validation: python3 check-peps.py --detailed peps/pep-0831.rst

Closes #5073.

• Change is either:
  • To a Draft PEP
  • To an Accepted or Final PEP, with Steering Council approval
  • To fix an editorial issue (markup, typo, link, header, etc)
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

[python-cla-bot]

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• cc0006@163.com

[read-the-docs-community]

Documentation build overview

📚 pep-previews | 🛠️ Build #35204955 | 📁 Comparing c64be22 against latest (6068058)

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1 file changed

± pep-0831/index.html

[StanFromIreland]

This is not an editorial issue, and this is a finalised PEP. In addition, the CLA is unsigned. As such, I'm closing this.