[Development] Switch the main "Qt Build System"

[Alexandru Croitor]

> On 9. Jun 2020, at 09:27, Shawn Rutledge <Shawn.Rutledge at qt.io> wrote:
> 
> 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.

Joerg already answered about this.

At least in my opinion, I think we should keep configure script because it's a nice developer interface. Some mappings are currently missing, but people could help us out there.

Indeed currently you need to run CMake at least once to get all the checkboxes. I have an idea of how we might improve that (so you don't have to run it once) but it's not done yet.

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

I think we should keep it.

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

Yes. It's a bug if it doesn't.

> 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?

You can use $qt_prefix/bin/qt-cmake-standalone-test to configure a single test (or subdir of tests), and build with ninja.
> 
> 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.

I agree.

> 
> Will we keep using the configure.json files and generating configure.cmake from them? 


It's possible, but it might make sense to to use configure.cmake as the source of truth as well. I don't believe we discussed that internally yet.