[Development] QT_WARNING_MSG/QT_DEPRECATED_HEADER

[Marc Mutz]

Hi,

I've developed these to deprecate qspan_p.h in favour of qspan.h, but 
we'll now just remove the old header instead. I can see us apply it to 
qglobal.h, but not for 6.7.

But if you have a use for a cross-platform #warning or the deprecation 
of a whole header, restore and stage 
https://codereview.qt-project.org/c/qt/qtbase/+/523363, it's already 
approved and just waits for a user.

Thanks,
Marc

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Marc Mutz <marc.mutz at qt.io>
Principal Software Engineer

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