preprocessor/7029: preprocessor should ignore #warning with -M

[Neil Booth]

Zack Weinberg wrote:-

> > Warnings are sent to stderr and the dpendency information isn't; so if
> > you don't want the warnings you are free to pipe stderr to /dev/null.
> > If you still wanted to retain errors but not warnings you can use -w,
> > or check the compiler return code.
> > 
> > So I disagree that there is any reason to change the existing behaviour,
> > and want to close this PR.  Do you agree, Zack?
> 
> Well, I think it might be reasonable for -M switches that inhibit
> compilation to imply -w.  Presumably the user will get all the
> warnings in due course as part of the real compilation.

Fair enough; I'll create a patch.

Neil.