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