https://bugs.python.org/issue25397

improve the cross-compile tests to be more focused

--- a/configure.ac
+++ b/configure.ac
@@ -1339,7 +1339,7 @@ if test "$GCC" = "yes"
 then
   AC_MSG_CHECKING(whether gcc supports ParseTuple __format__)
   save_CFLAGS=$CFLAGS
-  CFLAGS="$CFLAGS -Werror -Wformat"
+  CFLAGS="$CFLAGS -Werror=format -Werror=ignored-attributes"
   AC_COMPILE_IFELSE([
     AC_LANG_PROGRAM([[void f(char*,...)__attribute((format(PyArg_ParseTuple, 1, 2)));]], [[]])
   ],[
@@ -4458,7 +4458,7 @@ then
   [ac_cv_have_long_long_format="cross -- assuming no"
    if test x$GCC = xyes; then
     save_CFLAGS=$CFLAGS
-    CFLAGS="$CFLAGS -Werror -Wformat"
+    CFLAGS="$CFLAGS -Werror=format"
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
         #include <stdio.h>
         #include <stddef.h>