[Development] Switch the main "Qt Build System"

[Shawn Rutledge]

FWIW the configuration mechanism seems a bit less friendly so far with all those -DSHOUTED options like -DFEATURE_developer_build=ON instead of configure -developer-build.  But there is cmake-gui, which generates checkboxes for all the options after you have run cmake the first time.  They are even searchable.  Is that a recommended way to discover the configure options?  (Too bad you have to run cmake twice then.)

There will be no use for running the old configure script anymore then, right?

It will work from the top-level, so we can still do chained builds with all modules, right?

To build autotests, -DFEATURE_developer_build=ON will do that right?  And there are also BUILD_TESTING and QT_NO_MAKE_TESTS so what’s the relationship?

If you have done a build without tests or examples at first (for speed), and then you want to build some specific ones, how do you do that?

I see the same nice configure summary is still generated, but it would be nice to keep writing that to a file like the old configure system does, for later inspection.

Will we keep using the configure.json files and generating configure.cmake from them?  Then maybe we could write a better gui later on, that can show checkboxes before you run cmake the first time.  It could probably be a pure-qml implementation if we add a QProcess API (which a couple people have already tried to do over the years).