LOOKING across the sculptured gardens of his chateau, with its stone nymphs paying tipsy homage to the joy of intoxication, Paul-Henri de Bournazel explained that his ancestors had been making wine here for more than 400 years. ''It gives you strength in the difficult years,'' he said. ''As though you can hear them saying 'Allez!' - 'Get to it'!'' But Bordeaux winemakers, steeped in centuries of history, are facing competition from vintners of an entirely different lineage - Chinese investors who are snapping up struggling chateaus.