Emit a warning if a prototype is not generated due to known limitations

[facchinm]

arduino-builder skips prototype generation for some particularly crafted functions to avoid unclear/difficult to debug compilation errors. Anyway, it could be useful to inform the user that the prototype has not been generated so we can provide better assistance in the forum (like Paul's library detection info)

see https://github.com/arduino/Arduino/issues/5103

[nickgammon]

Yes, a warning may help. It's a bit of an edge case I admit.