[dunecontrol] Document already present --no-cmake.

Once CMake will be default, people can deactivate it.

bin/dunecontrol

@@ -764,7 +764,8 @@ usage () {
     echo "OPTIONS:"
     echo "  -h, --help          show this help"
     echo "      --debug         enable debug output of this script"
-    echo "      --use-cmake     use cmake instead autotools for building"
+    echo "      --use-cmake     use CMake instead of autotools for building"
+    echo "      --no-cmake      use autotools instead of CMake for building"
     echo "      --module=mod    only apply the actions on module mod"
     echo "                      and all modules it depends on"
     echo "      --only=mod      only apply the actions on module mod"