[Development] Qt 6 co-installability with Qt 5

[Marius Kittler]

Am Dienstag, 27. Oktober 2020, 17:34:44 CET schrieb Thiago Macieira:
> Have we fixed it?
> 
> I do not plan on updating qtchooser for Qt 6. Qt 6 should not install any
> binary with the same name as Qt 5 did.

For the GNU/Linux target is seems to work when setting a few CMake variables: 
https://github.com/Martchus/PKGBUILDs/blob/master/qt6-base/default/PKGBUILD

For the Windows target that counts as well besides the "qtmain" problem. I 
used so far the following patch to workaround it:
https://github.com/Martchus/PKGBUILDs/commit/
87c7401193391438554de35f1a8b062dcf59bd20#diff-484f0d9d749b6b5fdde1f91fd329a93c73e409defb4a494c4d90b3f08590d330
It seems like this b5af1408099dedd132f36e04d19cb5771a23ec28 "fixed" the 
problem. Actually, it isn't really fixed because when Qt 7 comes out there 
will be the same issue again.

For the Android target there were previously issues when using an installation 
layout like in my GNU/Linux example. When I remember correctly androiddeployqt 
would work otherwise so one needed to resort to an installation layout which 
would likely prevent installing Qt 5 alongside Qt 6. Not sure whether this is 
still relevant because I haven't tried building Qt 6 for Android yet.