Include externals as system headers

[Minty-Meeo]

Following a suggestion given by @iwubcode on PR #11709, let's just include all externals as system headers so the warnings from their headers can stop leaking into our warnings.

[Minty-Meeo]

Okay seems like the buildbots are not on CMake 3.25+

[iwubcode]

For VisualStudio, you can likely use: https://learn.microsoft.com/en-us/cpp/build/reference/external-external-headers-diagnostics?view=msvc-170

[OatmealDome]

Requiring CMake 3.25+ is problematic. Android Studio only comes with CMake 3.22.1 maximum (or at least that's what my copy is telling me).

[Minty-Meeo]

Curse Android, always holding back my PRs.

[shuffle2]

For VisualStudio, you can likely use: https://learn.microsoft.com/en-us/cpp/build/reference/external-external-headers-diagnostics?view=msvc-170

this is already implemented in msbuild so should just be able to copy the cmdlines it generates

note that msvc has also has concept of "system" headers as opposed to just "external". at least on msbuild, system is the wrong choice because msbuild does not do full change detection on system headers (which it considers part of the toolchain). so you may want to ensure cmake doesn't have the same semantics.