Add -Werror to build_warnings

[Paul Schlie]

> From: Zack Weinberg <zack@codesourcery.com>
> Would you please write complete sentences?  Your telegraphic style is
> extremely difficult to read.
>> Paul Schlie <schlie@comcast.net> writes:
>> - Fully understand your position in general, however honestly question the
>>   utility of the specific warning: of a comparison between two pointers to
>>   otherwise equivalent rank data types which only in sign-ness; as it's not
>>   clear that such a comparison could ever lead to unintended consequences,
> 
> You are mistaken.  We flushed all these warnings out of GCC about a
> year ago, and we found real bugs.  Mostly, if I recall correctly, they
> were cases where the two pointers should never have been being
> compared in the first place, and this was the only diagnostic you got.

- OK, I accept that it basically forces you to at least look; at which time
  you may discover something that wasn't intended somewhere. Which one could
  argue is better than nothing.

Thanks,