[Development] Switch the main "Qt Build System"

[Shawn Rutledge]

> On 2020 Jun 9, at 10:21, Alexandru Croitor <alexandru.croitor at qt.io> wrote:
> 
>> 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.

Well that’s a little extra maintenance work then; I agree that configure is nicer, but maybe we can expect the cmake way of configuring to generally be more up-to-date if that’s where each change starts.

> 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.

Do you want to explain?

>> 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.

At least json is easier to parse, if we try to write other tooling.