build: Set macOS minimum version to Qt min version

build: Set macOS minimum version to Qt min version

Set the macOS Info.plist minimum version to the minimum version
supported by Qt for macOS.

Determine the minimum macOS version for Qt through querying qmake.
Unfortunately, this involves a somewhat roundabout process of a fake
project file and searching qmake's output.

See https://code.qt.io/cgit/pyside/pyside-setup.git/tree/qtinfo.py?h=5.6

Switch to HFS+ (from APFS) so older macOS versions can parse the app
bundle and display the "OS version too old" warning, instead of just
warning about a corrupt image file.

Credit to freenode/Deas for point this out and providing screenshots!

See https://doc.qt.io/qt-5/macos.html
And https://doc.qt.io/qt-5/macos-deployment.html