[Interest] Tweaking a static build of Qt 6 for Windows

[Marius Kittler]

When it comes to building a static version of Qt itself then you need to 
adjust things on CMake-level, indeed. When your project is still using qmake, 
then qmake's "mkspecs" are supposedly still relevant for building your 
project. (I cannot say much about that because I'm using CMake in all my 
projects anyways.)

Note that my experience with Qt6/CMake was actually much better compared to 
Qt5/qmake. I had much less additional patching to do to make it work in my 
build environment.

However, there were still a few places that needed patching. This branch 
contains patches that I came up with for the mingw-w64 and GNU/Linux targets 
and a build environment where also most of Qt's own dependencies are provided 
as static libraries:
https://github.com/Martchus/qtbase/commits/6.4.2-mingw-w64
(Maybe you need to do things differently for the targets and build environment 
you're concerned with. I'm merely sharing those patches as they show the 
interesting "spots" in Qt's code. Additionally, not all patches on that branch 
are related to making a static build. There are also other branches in that 
repo based on earlier Qt releases. If you go back to Qt5 you'll see how it 
looked for me when Qt was still built with qmake.)

Best Regards
Marius