... CPP states results might be unknown if the parsed file is not in C format.

I'm not sure what this refers to. In my understanding, cpp is at root a text macro processor. It may have a number of features adapting it to C, C++ and ObjectiveC/C++ file processing, but these may be easily turned off. Strictly to satisfy my curiosity, can you give a reference to or example of the warning you mention?

... if I remember correctly, m4 doesn't handle
 #if defined (ABC) or defined(DEF)

I am not very familiar with m4, but I think you may be right, at least in that form. But if you can accept  || in place of  or as your logical operator, cpp would do the trick.

In any event, you seem to know what you want, so let me wish you every success in your quest.