-Wformat is needed by gcc 4.8 (closes #17547)
diff --git a/configure.ac b/configure.ac
index 6d54b16..9179fe6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1331,7 +1331,7 @@
 then
   AC_MSG_CHECKING(whether gcc supports ParseTuple __format__)
   save_CFLAGS=$CFLAGS
-  CFLAGS="$CFLAGS -Werror"
+  CFLAGS="$CFLAGS -Werror -Wformat"
   AC_COMPILE_IFELSE([
     AC_LANG_PROGRAM([[void f(char*,...)__attribute((format(PyArg_ParseTuple, 1, 2)));]], [[]])
   ],[