[3.10] bpo-41282: Fix broken make install (GH-26329)

[miss-islington]

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.

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.

This commit introduces 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'.

Co-authored-by: Ned Deily nad@python.org
(cherry picked from commit 563bd5a)

Co-authored-by: Petr Viktorin encukou@gmail.com

https://bugs.python.org/issue41282

Automerge-Triggered-By: GH:ned-deily

[miss-islington]

@encukou and @ned-deily: Status check is done, and it's a success ✅ .

[miss-islington]

@encukou and @ned-deily: Status check is done, and it's a success ✅ .

[miss-islington]

@encukou and @ned-deily: Status check is done, and it's a success ✅ .