develop: #warning preprocessor causes compile error on MSVC

[sevenc-nanashi]

MSVC does not support #warning, but DPF uses #warning in some places, thus the compile warning causes compile error on MSVC.

[falkTX]

we need to have better CI tests for msvc to be able to detect these things. I dont use it myself, so need contributions to that.
but there is a minimal set of CI jobs with msvc already, and they dont fail... isnt invalid pragmas a warning? maybe you have warnings as errors?

[sevenc-nanashi]

minimal set of CI jobs with msvc already, and they dont fail

I think it's because all examples are correct.
I encountered this situation by #defineing DISTRHO_PLUGIN_HAS_EMBED_UI as 1 on develop branch.
This situation activated this branch: https://github.com/DISTRHO/DPF/blob/develop/distrho/src/DistrhoPluginChecks.h#L146-148 , which includes the #warning preprocessor.