Show usage and abort if an unknown option is passed to configure (Carlo Marcelo Arenas Belon).

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@3666 c046a42c-6fe2-441c-8c8c-71466251a162

configure

@@ -316,6 +316,8 @@ for opt do
   ;;
   --disable-werror) werror="no"
   ;;
+  *) echo "ERROR: unknown option $opt"; show_help="yes"
+  ;;
   esac
 done