Disable -Werror when error attribute generates warnings

[Grégoire Sutre]

Hi,

With an older version of gcc that does not understand the error 
attribute, gcc generates warnings when compiling files that include 
include/grub/list.h.  Since TARGET_CFLAGS contains -Werror by default, 
the build of modules fails.

The following patch checks whether the C compiler supports the error 
attribute without warning, and disables -Werror if that is not the case 
(as otherwise the build will fail).

http://pkgsrc-wip.cvs.sourceforge.net/viewvc/*checkout*/pkgsrc-wip/wip/grub2-current/patches/patch-gcc-warning-on-error-attribute

This is merely a suggestion.  I'm no autoconf expert, and I'm not even 
sure that this kind of check is a good idea, as configure has the option 
--disable-werror that can be used anyway.

Grégoire

p.s. I did not see this problem when I tested the patch discussed in the 
thread [1] as I focused on building utilities at that time, sorry.

[1] http://lists.gnu.org/archive/html/grub-devel/2009-12/msg00324.html