Disabling unity for subprojects

[epsilon-phase]

It does not seem to be possible to create a subproject that is not built as a unity build when the main project is.

This is inconvenient, since the subproject is a much larger dependency than my code base and I have little interest in fixing the issues that result from it being a very C project.

Is it possible to resolve this currently? And if not, would it be possible to introduce a per-project unity build setting?

[bonzini]

Fixed by #14251.