Skip Python installation paths for build-tree Python

[jameshilliard]

When building against an uninstalled Python, LIBDIR and INCLUDEDIR can describe the eventual installation rather than usable build-time paths. The sysconfig-based lookups added in #7634 can therefore add unprefixed paths such as /usr/lib and /usr/include when cross-compiling.

Changes proposed in this pull request:

• Guard the two additional Python installation-directory lookups with sysconfig.is_python_build(), leaving Python header and library configuration to setuptools.
• Preserve the existing sysconfig lookups and sys.prefix fallbacks for installed Python.
• Add a release note.

Testing

• Built and installed a native CPython 3.14 wheel.
• python3 selftest.py: 59 tests passed.
• Focused tests covering sanity checks, features and image operations: 210 passed, 2 skipped.
• Dependency detection, without compilation, checked with installed native Python, uninstalled native Python and an AArch64 cross-compilation environment. JPEG, zlib, TIFF, OpenJPEG, FreeType, LittleCMS and WebP were detected successfully in each environment.
• Eight focused guard checks passed, covering build-tree detection, virtualenvs and missing configuration values.
• Pre-commit checks and release-note validation passed.

[chewi]

Successfully tested on Gentoo. Thank you. Sorry for the fallout.