Avoid calls to the undeclared exit function.  This helps with
compatibility with future compilers that reject implicit function
declarations by default.

Not upstreaming this because the actual fix is in autoconf, and
re-running autoconf makes the issue go away.

diff --git a/configure b/configure
index 69353571c1676bca..67f8ec12e8ecc7b9 100755
--- a/configure
+++ b/configure
@@ -2514,7 +2514,7 @@ if { (eval echo "$as_me:$LINENO: \"$ac_compile\"") >&5
   echo "$as_me:$LINENO: \$? = $ac_status" >&5
   (exit $ac_status); }; }; then
   for ac_declaration in \
-   '' \
+   '#include <stdlib.h>' \
    'extern "C" void std::exit (int) throw (); using std::exit;' \
    'extern "C" void std::exit (int); using std::exit;' \
    'extern "C" void exit (int) throw ();' \
@@ -6906,8 +6906,8 @@ main ()
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext