The Boston Red Sox, who had jumped out as the clear favorite to acquire White Sox left-hander Mark Buehrle during the weekend, have pulled out of the sweepstakes, according to one source. And not because the asking price of top minor-league prospects was too high or because they no longer coveted Buehrle's services, but because Buehrle is no longer on the trading block for the time being. The Sun-Times has learned that a change of heart in both the Buehrle and White Sox camps during the last 48 hours has led to talks about the sides somehow getting together and agreeing on a contract extension by the end of the week. That sentiment also was expressed by one Sox player, who said the ''climate'' was right for Buehrle to sign an extension in the next few weeks. ''Call it a calm before the storm,'' the player said.