[Interest] Require a qt_finalize_target() call in CMake user projects that use a static Qt

[Marius Kittler]

> Is your concern about the plugins packages being automatically found and
> targets are automatically created (so that find_package is run at all), or
> that they get linked to the final executable / shared library?

I have generally no problem with targets being made available. However, if it 
means installing e.g. PostgreSQL in the build environment only to find a 
dependency of a plugin I don't want to link against anyways that might be a 
bit anoying. Nevertheless, I am mainly concerned about which plugins my final 
executable / shared library is linked against.

> And is it about generic qt plugins and/or qml plugins?
> qml plugins are not linked in by default

>From my user's point of view I don't make a difference. I don't remember 
exactly the problem with the plugins included without 
QT_SKIP_AUTO_QML_PLUGIN_INCLUSION. Maybe it wasn't really that they would have 
been linked against but just the CMake error about the missing PostgreSQL 
library.