Don't use -Wno-error=undef

[Joseph Myers]

I've now committed this patch, known to work on x86_64, x86, arm and 
mips64 (and Mike tested some other architectures).

I expect any remaining architecture-specific issues will be 
straightforward to fix, but if the right way to fix something is unclear 
then ask on libc-alpha and I'll try to work out an appropriate solution 
for the given case.

-- 
Joseph S. Myers
joseph@codesourcery.com