Add -Werror to build_warnings

[Alan Modra]

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.

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre