VREND_DEBUG_ENABLED represents if debugging features are enabled and is
controlled by NDEBUG macro. By using VREND_DEBUG_ENABLED in
an if statement instead of using NDEBUG in ifdef, the compiler can
validate the content of the conditional and know its variable usage to
avoid meaningless warnings.
Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>