fix(windows): Support MSVC Debug build linking against Release Python

[jcfr]

This PR fixes Windows/MSVC build and link issues when PythonQt is built in Debug while linking against a Release Python (e.g., only pythonXY.dll/pythonXY.lib are available).

Summary:

• Adopt the strategy used in VTK to temporarily undefine _DEBUG only for the Python.h inclusion, while:

  • Pre-including low-level CRT headers before undefining _DEBUG to avoid pulling in release-mode CRT variants inadvertently.
  • Defining _CRT_NOFORCE_MANIFEST and _STL_NOFORCE_MANIFEST on MSVC ≥ 1400 to prevent inconsistent CRT manifest flags.
  • Restoring _DEBUG afterward to keep the rest of the TU in the correct debug configuration.

• Wrap the behavior with a small guard macro:

  • Introduce PYTHONQT_UNDEF_DEBUG so we can safely re-define _DEBUG only when we actually changed it.

• All changes are conditioned on PYTHONQT_USE_RELEASE_PYTHON_FALLBACK && _DEBUG.

Motivation:

Without these adjustments, MSVC may:

• Emit CRT manifest inconsistencies (e.g., _CRT_MANIFEST_DEBUG, _CRT_MANIFEST_RETAIL, _CRT_MANIFEST_INCONSISTENT) across the same Translation Unit.
• Trigger link errors such as _invalid_parameter_noinfo_noreturn due to mismatched debug/release CRT resolution caused by including system headers with the wrong _DEBUG state.

References:

• Mirrors the approach in VTK: Kitware/VTK@81d4a72
• Originally introduced via commontk/PythonQt@8a50eaa6 and refined in commontk/PythonQt@77d8177.
• Original issue: PythonQT Fails to Build in Debug on MSVC 2010 commontk/PythonQt#9 (thanks @Neosettler).

[jcfr]

Suggested commit message if squashing:

fix(windows): Support MSVC Debug build linking against Release Python

This changes how we include `Python.h` on Windows when
`PYTHONQT_USE_RELEASE_PYTHON_FALLBACK` is enabled and `_DEBUG` is set:

* Pre-include low-level CRT/system headers **before** undefining `_DEBUG` to
  avoid pulling release-mode CRT variants in a debug TU.
* Temporarily undefine `_DEBUG` only for `Python.h`, then restore it using a
  `PYTHONQT_UNDEF_DEBUG` guard.
* On MSVC >= 1400, define `_CRT_NOFORCE_MANIFEST` and `_STL_NOFORCE_MANIFEST`
  to prevent manifest inconsistency flags.

This prevents CRT mismatch issues and link errors like
`_invalid_parameter_noinfo_noreturn` when building PythonQt in Debug against a
Release Python.

References:
* kitware/VTK@81d4a72c1195437403f1772559c9c28978818f03
* commontk/PythonQt#9

(cherry-picked from commontk/PythonQt@8a50eaa6, commontk/PythonQt@77d8177)

Co-authored-by: John Stark <John.Stark@inria.fr>
Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

[mrbean-bremen]

Looks good!