ci: treat CMake deprecated and development warnings as errors

[madebr]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

I'm not 100% sure we're not using any deprecated feature accidentally (e.g. through CMake toolchain files)

Description

Existing Issue(s)

Fixes #16007

[slouken]

I think the idea is to fail on all warnings, and if we run into a warning we can't fix, then we exclude that one specifically.

[slouken]

Or is this for CMake and not the compilation itself?

[madebr]

This is for CMake only.

SDL itself is built with -Werror for practically all platforms.
Only powerpc, riscos, ngage and djgpp are not warning-free.

[madebr]

ci is green, so good to merge.
See https://github.com/madebr/SDL/actions/runs/29462303599

[sezero]

I think SDL2 branch needs it too.

[madebr]

I think SDL2 branch needs it too.

Indeed. I removed the -Werror option there.
86a6368

[slouken]

Thanks!