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

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>

Lib/sysconfig.py

@@ -182,6 +182,18 @@ def is_python_build(check_home=False):
 
 _PYTHON_BUILD = is_python_build(True)
 
+if _PYTHON_BUILD:
+    for scheme in ('posix_prefix', 'posix_home'):
+        # On POSIX-y platofrms, Python will:
+        # - Build from .h files in 'headers' (which is only added to the
+        #   scheme when building CPython)
+        # - Install .h files to 'include'
+        scheme = _INSTALL_SCHEMES[scheme]
+        scheme['headers'] = scheme['include']
+        scheme['include'] = '{srcdir}/Include'
+        scheme['platinclude'] = '{projectbase}/.'
+
+
 def _subst_vars(s, local_vars):
     try:
         return s.format(**local_vars)