[Qbs] Suggested way to add non-default build variants

[Epting, Thomas]

> > What is the suggest way to add non-default build variants in Qbs?
> > Example: In addition to debug and release, we want to support a build variant releaseWithDebugInfo.
>
> I don't think this is actually true. In particular, if have no idea what "add more in a project file" means.
> The buildVariant property is used for two things:
> 1) A number of other properties derive their default values from it, e.g.  cpp.optimization. These can
> of course be overwritten, so in effect you have a custom build variant then.
> 2) There are several places where we assume one of "debug" and "release", for instance when
> deciding which variant of the Qt libraries to link against. Therefore, I think you need to go with one
> of the two predefined values.

Thanks.

I also found QBS-88 which seems to address what we need.

Thomas