I'm not sure if there's an overarching epic for "build Qt with more and more warnings enabled by default" where to discuss which warnings we want to get enabled (possibly even discussing regular vs. developer builds).

This is just about -Wundef, which should be enabled unconditionally when building Qt. A typo when checking for a macro goes otherwise unnoticed and introduces bugs.

-Wundef is already enabled for headersclean, we should extend it to building the whole of Qt.