An 81-year-old man was found dead on a Brooklyn apartment building rooftop, police said Sunday. The victim’s body was discovered on top of the six-story apartment building on Avenue H near New York Ave. in Flatbush about 9:55 a.m. Saturday, cops said.  His name was not immediately released. Polic e could not say Sunday whether his death is believed to be cold related or what he was doing on the roof. As of Saturday, 17 people have died citywide amid the historically cold temperatures that have gripped the city since Jan. 24. ...read more read less