CMakeLists.txt: adjust cmake build rules for Qt6 support

Check for the availability of Qt5 as well as Qt6. Prefer Qt5 for
backwards compatibility, but accept when only Qt6 is available.
This unbreaks Mac OSX 12 builds with homebrew.

This change is based on work that was submitted by Dominik Sliwa
<redacted>.

It is assumed that failed tests for timestamps to text conversion
(the format_time_minutes() routine) and Qt widgets not being thread
safe are issues that do reproduce more often on the Mac platform
but are independent from Qt6 support. These issues will be dealt with
separately.