If QT_VERSION is not defined, it gets define with the AUTO value. And
its definition is moved to the helper file.
find_qt now:
- Check quietly for Qt5 and Qt6
- If QT_VERSION is set to AUTO. It checks firstly if Qt5 was found it
will use it. If not it do the same for Qt6
- If QT_VERSION is set to 5 or 6, it checks if the choice was found and
use it. And if not, it falls back to the other if found.
- If neither Qt5 or Qt6 are found, a fatal error is emitted.
- The macro saved the _QT_VERSION in the cache to replace QT_VERSION,
so the process is not repeated each time that find_qt is used.
- When Qt::Gui is in the Linux component list, Qt::GuiPrivate is added.
So using the versioned one is no longer required.
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