Avoid an implicit declaration of exit in the configure script.  This
ensures that the package continues to build with future compilers
which do not support such implicit declarations by default.

diff --git a/configure b/configure
index 8fd0eed37cb997de..f5ebc89db38d88f4 100755
--- a/configure
+++ b/configure
@@ -4323,8 +4323,8 @@ 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$ac_exeext
@@ -18747,8 +18747,8 @@ main ()
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext