Replace a C99-incompatible code fragment generated by autoconf.

No need to upstream this because a new upstream release with

newer autoconf will likely address this issue.

diff --git a/configure b/configure

index d797a7c8684821f7..a3e8661e25f3cc10 100755

--- a/configure

+++ b/configure

@@ -3841,8 +3841,8 @@ 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