Migrate Windows build system to CLANG64

[jmckisson]

Since msys2 is deprecating MINGW64, a change to CLANG64 is needed

[vadi2]

Nice work parameterizing QT_VERSION throughout — but the generated CMake shim files for Qt6BundledPcre2 and the other bundled libs (Zlib, Freetype, etc.) in build-win.sh still hardcode "6.9.1". These should use ${QT_VERSION} to stay in sync.

---

Minor suggestion in setup-win-sdk.sh: if pacman -Q produces unexpected output, QT_VERSION could end up empty, leading to a confusing git clone --branch v failure downstream. A quick guard after the detection line would make this more robust:

if [ -z "$QT_VERSION" ]; then
  echo "ERROR: Could not detect Qt version from pacman"
  exit 1
fi

[jmckisson]

Ah good catch, will fix later today at some point, or tomorrow because Easter