configure: Use -Wundef if supported by the compiler.

[Gerald Pfeifer]

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.

Gerald
---
 configure.ac |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/configure.ac b/configure.ac
index eb166ee..3587e13 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1668,6 +1668,7 @@ then
   WINE_TRY_CFLAGS([-Wdeclaration-after-statement])
   WINE_TRY_CFLAGS([-Wstrict-prototypes])
   WINE_TRY_CFLAGS([-Wtype-limits])
+  WINE_TRY_CFLAGS([-Wundef])
   WINE_TRY_CFLAGS([-Wwrite-strings])
 
   dnl Check for noisy string.h
-- 
1.7.2.2