OK, finally solved the problem with QT5, though without being really clear what worked or why. It did finally compile and installed itself politely in /opt.
Now there is another puzzle. The problem was to install and run sigil. Which is a very nice package indeed if you want to edit epub books.Very nice. Unfortunately you have to compile it from source. They do not do binaries, and if they have as hard a time as me with it, I can understand why.
The problem is that entering the command sigil in a terminal produces an error to the effect that libQt5 widgets is not found.
However, the binary is installed in my home directory in a sigil folder, and if I execute this binary it works just fine.
If I look in the /opt sigil directory, qt5widgets is indeed there.
So how do I change the command so that it calls the binary that works? There must be a file someplace that has all these commands in it, but where?
Al