Avoid an undeclared exit function call in obscure configure check.

This improves compatibility with strict(er) C99 compilers, which may

not support implicit function declarations because they were removed

from the C language in 1999.

Not submitted upstream as regeneration of configure with new autoconf

would fix.

diff --git a/configure b/configure

index b81fdff..49bcfc6 100755

--- a/configure

+++ b/configure

@@ -3850,8 +3850,8 @@ 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