diff --git a/configure.ac b/configure.ac
index fe389ebb35..628e0c8afc 100755
--- a/configure.ac
+++ b/configure.ac
@@ -295,6 +295,7 @@ AC_C_FLAG([-Wmissing-declarations])
 AC_C_FLAG([-Wpointer-arith])
 AC_C_FLAG([-Wbad-function-cast])
 AC_C_FLAG([-Wwrite-strings])
+AC_C_FLAG([-Wundef])
 if test "$enable_gcc_ultra_verbose" = "yes" ; then
   AC_C_FLAG([-Wcast-qual])
   AC_C_FLAG([-Wstrict-prototypes])