show usage and abort if unknown option is passed to configure

The following patch prints an error if an unknown option is passed to configure and directs the script to show usage information.

Carlo

---
Index: configure
===================================================================
RCS file: /sources/qemu/qemu/configure,v
retrieving revision 1.165

diff -u -r1.165 configure
@@ -306,6 +303,8 @@
         *)     echo "undefined SPARC architecture. Exiting";exit 1;;
       esac
   ;;
+  *) echo "ERROR: unknown option $opt"; show_help="yes"
+  ;;
   esac
 done