[Development] Switch the main "Qt Build System"

[Alexandru Croitor]

Hi,

The CI still uses qmake to build the examples projects, to make sure that we don't lose coverage of using qmake for application development (which is different from building Qt).

That's been tested to work for all repos that are part of qt5, but i guess we missed checking the addon libraries like qtimageformats or qt3d. 

Please file a bug report : )

> On 11. Dec 2020, at 12:37, Mike Krus <mike.krus at kdab.com> wrote:
> 
> Some configurations still seem to use qmake for some parts:
> https://testresults.qt.io/logs/qt/qt3d/5ef6de3b5deb13254200c1f385ce09008b287760/MacOSMacOS_10_14x86_64MacOSMacOS_10_14x86_64Clangqtci-macos-10.14-x86_64-f695c4DisableTests_Sccache/67c64555fe39a6412f13f7c4d92663fc0c175a7b/build_1607682919/log.txt.gz
>