Allow incompatibility notification

[Drup]

In a perfect word, if_impl would solve that. However, apparently according to the mirage impl, it doesn't. :)

One way could be to implement a configurable that always fails (and then if_impl is on the tracks again !). The issue is that it clutters the config graph a bit.

See mirage/mirage#429

[yomimono]

I think this also required to fix mirage/mirage#525 .

[Drup]

I think the two are related but distinct. This bug report is about incompatible devices. @sgrove want to be warned about incompatible libraries (which probably involve some ocamlfind magic; but not much functoria things).