configure: Use -Wundef if supported by the compiler.

[Gerald Pfeifer]

On Mon, 3 Jan 2011, Alexandre Julliard wrote:
>> I verified this does not cause any warning on FreeBSD 8.1 test
>> builds, and all the tools like bison and flex in somewhat current
>> versions.
> It's broken here with bison 2.4.1:
> 
> make[1]: Entering directory `/home/julliard/wine/wine/libs/wpp'
> gcc -m32 -c -I. -I. -I../../include -I../../include   -D_REENTRANT -fPIC -Wall -pipe -fno-strict-aliasing -Wdeclaration-after-statement -Wstrict-prototypes -Wtype-limits -Wundef -Wwrite-strings -Wpointer-arith -Werror  -g -O2  -o ppy.tab.o ppy.tab.c
> ppy.tab.c:336:6: error: "YYENABLE_NLS" is not defined
> ppy.tab.c:910:6: error: "YYLTYPE_IS_TRIVIAL" is not defined
> make[1]: *** [ppy.tab.o] Error 1

This does not occur with bison 2.4.3.  I investigated, and it seems
this was actually fixed with bision 2.4.2:

http://git.savannah.gnu.org/cgit/bison.git/commit/?id=3d04b5673d26813f3fcec37982ee84d05c3d71aa

Checking this via autoconf sounds a bit involved.  Do you have a
recommendation?  Perhaps just wait until "Alexandre runs bison 2.4.2
or later on all his machines"? ;-)

Gerald