-Werror now enabled by default

[Joseph Myers]

I've committed the patch to enable -Werror by default.  I get a clean 
build and test on x86_64 with GCC 4.9, but *this is expected to break the 
build for some other configurations*.  If you see build breakage from 
warnings, you should submit patches to fix or disable those warnings as 
appropriate (or use -Wno-error if really necessary - only for warnings not 
enabled by an option that can be used in diagnostic pragmas).

-Wno-error=undef is used until the build and test is clean for -Wundef on 
at least one configuration.

-- 
Joseph S. Myers
joseph@codesourcery.com