bpo-41282: Fix broken make install

[encukou]

A previous commit broke a check in sysconfig when building cpython itself.
This caused builds of the standard library modules to search a wrong
location (the installed location rather than the source directory) for
header files with the net effect that a make install
incorrectly caused all extension modules to be rebuilt again and
with incorrect include file paths.

There's a complication. When building Python, we need two distinct
"include" directories:

• source .h files
• install target for .h files

Note that this doesn't matter except when building Python from source.

Historically:

• source .h files were in the distutils scheme under 'include'
• the install directory was in the distutils.command.install scheme
  under 'headers'

GH-24549 merged these; sysconfig is now the single source of truth and
distutils is derived from it.

Introduce a "secret" scheme path, 'headers', which contains
the install target. It is only present when building Python.
The distutils code uses it if present, and falls back to 'include'.

https://bugs.python.org/issue41282

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit b300aac 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[frenzymadness]

Your analysis makes sense, the code looks good and I'm able to identify why I've never met this problem before when I was preparing the merge of sysconfig modules.

I'm sorry for the mistake I made.

[ned-deily]

This looks good to me and a more robust one than my initial stab at it. Thanks, @encukou! If you don't get to it soonish, I'll probably merge this to get the backport to 3.10 going.

[miss-islington]

Thanks @encukou for the PR, and @ned-deily for merging it 🌮🎉.. I'm working now to backport this PR to: 3.10.
🐍🍒⛏🤖

[bedevere-bot]

GH-26336 is a backport of this pull request to the 3.10 branch.