RFC: Erroring out if the platform you're building the Framework is not supported

[Allen Winter]

On Wednesday, February 16, 2022 9:42:18 AM EST Albert Astals Cid wrote:
> Supported == listed in metainfo.yaml
> 
> https://invent.kde.org/frameworks/extra-cmake-modules/-/merge_requests/245
> 
> What do you think?
> 
> I know parsing yaml like that isn't great but it seems to work for something as simple as what I want there.
> 
> I guess it's probably a bit annoying for places like OpenBSD where having Linux and FreeBSD supported probably means it would also work, but IMHO they can either just use -DIGNORE_PLATFORM_CHECK=true or work with us so we get an OpenBSD CI and then we add it to the metainfo.yaml list.
>
Here's why I asked Albert about this:
- for a long time kdesu builds fine on Mac
 - suddenly it doesn't build any longer
 - I spend time investigating and report a build failure
 - Developer (in this case Ahmad) spends time looking
    - responds that the metainfo.yml says kdesu is only for Linux and FreeBSD
    - Allen says "never mind then"
 - Today another person notices that kdesu no longer builds for Mac
   => repeat ad nauseam

So, to save everyone's time I propose that the CMakeLists.txt notifies 
the user explicitly if project is not supported on current platform.