Output the error message for unsupported options

While CONFIG_MULTITHREAD = 0, threads > 1 is not supported. Ouput
the error message.

Change-Id: I4e2aceb8d9574c76fd3e721b7cef9836fc2bb95f
diff --git a/apps/aomdec.c b/apps/aomdec.c
index 834f161..e1741fd 100644
--- a/apps/aomdec.c
+++ b/apps/aomdec.c
@@ -604,6 +604,13 @@
       summary = 1;
     } else if (arg_match(&arg, &threadsarg, argi)) {
       cfg.threads = arg_parse_uint(&arg);
+#if !CONFIG_MULTITHREAD
+      if (cfg.threads > 1) {
+        die("Error: --threads=%d is not supported when CONFIG_MULTITHREAD = "
+            "0.\n",
+            cfg.threads);
+      }
+#endif
     } else if (arg_match(&arg, &rowmtarg, argi)) {
       row_mt = arg_parse_uint(&arg);
     } else if (arg_match(&arg, &verbosearg, argi)) {