[Development] Build system for Qt 6

[Kyle Edwards]

On Fri, 2018-12-07 at 16:40 -0500, Matthew Woehlke wrote:
> I'm not sure if this belongs in CMake *itself*, but I think having a
> script (Python? Bash? Other?) to do it would be a great idea.

I think it would make sense to write the generation logic in a cmake -P
script. That way it would work on Windows, Linux, and Mac and require
only the tools that they already have to have anyway to build (CMake-
ified) Qt. The cmake -P call could optionally be wrapped in a .bat and
.sh file to make it slightly more pleasant to work with.

Kyle