encompass Qt6 support in the common hints file macOS.2020 instead
of a separate macOS.2020-qt6, which had already diverged and would
likely be a maintenance headache.
From the Makefile perspective:
WANT_WIN_QT=1 will default to Qt5
WANT_WIN_QT5=1 will assume Qt5
WANT_WIN_QT6=1 will assume Qt6
You must have the appropriate Qt installed, of course.
I expect some additional fine-tuning will be needed as this gets
tried out.
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