HOW come John McCain is having a fund-raiser at billionaire Ron Perelman‘s Creeks estate in East Hampton on July 19, even though Perelman backs Barack Obama? Turns out Perelman is lending the Republican presidential nominee his home to fulfill the wishes of the Revlon mogul’s closest business ad viser and friend of 40 years, McCain backer Howard Gittis, who died last September. Gittis promised McCain he would hold an event in the Hamptons for the Arizona senator if he won the GOP nomination, but didn’t live to honor the commitment. Perelman’s name is not even on the fund-raising invitation. The event is co-hosted by Barry Schwartz, a vice chairman of Perelman’s holding company MacAndrews & Forbes, and real estate mogul William Mack. Perelman earlier supported Hillary Clinton and held a fund-raiser for her last summer. “Ronald remains a strong Democrat,” Perelman’s rep, Christine Taylor, told The Post’s Carl Campanile. “He’s supporting Senator Obama. If Senator Obama would like us to hold a fund-raiser, we’d be more than happy to.”