Don't use -Wno-error=undef

[Joseph Myers]

On Tue, 18 Aug 2015, Andreas Schwab wrote:

> There is no need for build bots, cross compilers exist.

I don't see that as of any significant relevance.  Standard practice is 
that patches are tested on one configuration (hopefully one to which the 
patch is relevant), not the large number (hundreds?) of configurations 
required to cover all variations of which bits of source code might or 
might not be enabled when compiling and which macros might or might not be 
defined.  We don't even have an equivalent of GCC's contrib/config-list.mk 
to test the build of lots of configurations (and if we did, I doubt the 
build results for it would be clean now, or would stay clean, without bots 
- and it would still be overkill to use it for pre-commit testing unless 
it's very quick).

-- 
Joseph S. Myers
joseph@codesourcery.com