[Interest] minimum macOS runtime version for Qt 5.9+

[Hamish Moffatt]

https://doc.qt.io/qt-5/supported-platforms.html lists the supported 
macOS environments, but it's not clear if it's the same versions 
(10.10+) for runtime (deployment) as well as development, or whether 
running on earlier versions is supported. Does anyone know?

http://doc-snapshots.qt.io/qt5-5.10/supported-platforms.html suggests 
that 5.10 will increase this requirement further to 10.11+. Why is this?



Thanks,

Hamish