Fix historic autoconf C99 incompatibility: avoid calling the undeclared

exit function.

No need to upstream this because the issue goes away once autoconf

is re-run.

diff --git a/configure b/configure

index 93c5b8f92f3aefb4..7b0bfc3085462591 100755

--- a/configure

+++ b/configure

@@ -2389,8 +2389,8 @@ main ()

   for (i = 0; i < 256; i++)

     if (XOR (islower (i), ISLOWER (i))

         || toupper (i) != TOUPPER (i))

-      exit(2);

-  exit (0);

+      return 2;

+  return 0;

 }

 _ACEOF

 rm -f conftest$ac_exeext