configure: ignore unknown --xyzdir options

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
diff --git a/configure b/configure
index 5eb3ecd..fa1b2d0 100755
--- a/configure
+++ b/configure
@@ -673,6 +673,8 @@
   ;;
   --enable-vhost-net) vhost_net="yes"
   ;;
+  --*dir)
+  ;;
   *) echo "ERROR: unknown option $opt"; show_help="yes"
   ;;
   esac