I'd prefer the use of code to determine if the platform is unsupported over trying to standardize a data format for such declarations [for example, you might declare a platform as unsupported because it lacks the quantumflux.h file or because the quantumflux.h files does not define entangle() or because there is no /proc/torhock present].

Plus, the use of code to make such determinations is already supported, it just isn't documented sufficiently conveniently.