HUGO GREENHALGH THE United States will elect an LGBT+ president, but it might take another 20 years before the rainbow flag flies metaphorically over the White House, according to former Congressman and veteran civil rights campaigner Barney Frank. Attitudes toward the LGBT+ community have shifted "significantly" since he was first elected to Congress from the northeastern state of Massachusetts in 1981, Frank said. "The country has made it very clear – full legal equality wins and prejudice loses," he told the Thomson Reuters Foundation in a telephone interview. "I believe the fight for LGBT rights has been won (in the…