Have an option for C++ warnings to be errors in cxx-qt-build

[ahayzen-kdab]

If warnings occur in our C++ compilation these are currently ignored, have a way (eg via a feature or cfg or env var) to enable this for CI and developers.

Note we probably don't want this on all the time, otherwise we may fail with different GCC/Clang version for app developers.

[LeonMatthesKDAB]

I think it's best to opt for a --cfg option. The issue with Cargo features is that they're additive, so if any of your transitive dependencies enables it, it's enabled for your whole app, which will likely cause unnecessary build failures, as there are new warnings added in GCC/Clang all the time.

So having it a --cfg option would make it easy for us to enable it in CI, e.g. pass --cfg cxx-qt-werror, and down-stream dependencies can enable it for their own build only.