Add MinGW (Windows GNU) toolchain support

[GuyFromTheInternet]

This PR adds support for building cxx-qt applications with the MinGW (x86_64-pc-windows-gnu) toolchain on Windows.

Changes:

Started using Q_COREAPP_STARTUP_FUNCTION for metatype registration, the previous implementation used std::call_once for one-time metatype registration during static initialization. This fails on MinGW because the pthread TLS emulation (_emutls*) used by MinGW's std::call_once implementation isn't initialized during static constructors, causing a crash.

I also use atomic operations for initialization guards since this is simpler, has no TLS dependencies and works correctly on all platforms including MinGW.

Files changed:

• crates/cxx-qt/src/init.cpp - Use Q_COREAPP_STARTUP_FUNCTION (Qt5 only)
• crates/cxx-qt-lib/src/core/init.cpp - Use Q_COREAPP_STARTUP_FUNCTION
• crates/cxx-qt-lib/src/gui/init.cpp - Use Q_COREAPP_STARTUP_FUNCTION
• crates/cxx-qt-build/src/lib.rs - Use atomic operations for initialization

Tested with Qt 6.8.3 MinGW build on Windows 11 using x86_64-pc-windows-gnu target.

[ahayzen-kdab]

Awesome! Thanks for looking at solving this!

Looks like CI is failing as clang-format needs to run over the files, are you able to do that?

And then would you be comfortable attempting to add a CI runner for this? As it would be good to ensure we do not regress and we can prove that it works.

This would be a matter of adding MinGW to the matrix similar to MSVC here (can be release rather than debug) https://github.com/KDAB/cxx-qt/blob/main/.github/workflows/github-cxx-qt-tests.yml#L394 set the values in the matrix, eg likely it is win64_mingw as the aqt_arch, then check if any other runner.os == 'Windows' need updating/changing. And note only do this for Qt 6, don't both with Qt 5.

[ahayzen-kdab]

And guess there might need to be a Rust toolchain installed too, so might need to be an additional entry for that

[GuyFromTheInternet]

Pretty sure I did everything you asked

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (55106f0) to head (ed42b48).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #1396    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           75        75            
  Lines        13124     13457   +333     
==========================================
+ Hits         13124     13457   +333     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.