bug#20536: avoid new gcc warning: ENOTSUP vs EOPNOTSUPP

[Paul Eggert]

Jim Meyering wrote:

+  return err == EOPNOTSUPP
+#if ENOTSUP != EOPNOTSUPP
+    || err == ENOTSUP
+#endif
+    ;

Would the following work instead?  It's a bit cleaner to avoid #if:

  return err == EOPNOTSUPP || (ENOTSUP != EOPNOTSUPP && err == ENOTSUP);