MinGW compilation warnings in libiberty's waitpid.c

[Eli Zaretskii]

> From: DJ Delorie <dj@redhat.com>
> Cc: gcc-patches@gcc.gnu.org, gdb-patches@sourceware.org
> Date: Tue, 23 May 2017 15:37:32 -0400
> 
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> > Instead of making waitpid an always-failing stub on MinGW, wouldn't it
> > be better to make it work on MinGW?  Like this:
> 
> That's up to you, if it's target-specific.

Then I prefer this solution.

> What about mingw64?

It will be covered as well, because it defines __MINGW32__, and
because _cwait is in the MS runtime, so available to MinGW64, too.

Is there anything else I need to do about this part to get it solved
in the upstream repository?

Thanks.