Add -Werror to build_warnings

[Paul Schlie]

(sorry, wrong subject in earlier message got a way from me)

> Alan Modra <amodra at bigpond dot net dot au>
> I think we could enable -Werror once someone approves my cgen patch.
> About all we're left with is a zillion gcc-4.0 warnings about
> "pointer targets ... differ in signedness".  So if Ben can figure out
> how to add -Wno-pointer-sign for compilers that support it, then -Werror
> should soon smoke out the last few compile warnings.

- and/or recommend that GCC-4.0 eliminate that frivolous warning by default.