If you’re going to throw yourself an 85th birthday party, why not invite some friends over for dinner and sing a few songs around the piano? That’s how Elaine Stritch celebrated, with a special perf Tuesday of her hit cabaret show “At Home at the Carlyle: Elaine Stritch Singin’ Sondheim … One Song at a Time.”“I pity any girl who isn’t me tonight,” deadpanned the Broadway baby in her opening number, “I Feel Pretty,” while the man himself, Stephen Sondheim, howled with laughter at a table a few feet away that included Harold Prince and Frank Rich. “In my day,” began Stritch in a recollection, before thinking better of it. “What the hell am I talking about? This is my day.” Among those in the house at the Cafe Carlyle were Patti LuPone, Nathan Lane, Michael Feinstein and Disney’s Thomas Schumacher. Also blowing out candles on her own birthday cake that evening was columnist Liz Smith at an all-blonde table of media mavens that included Leslie Stahl, Joni Evans, Cynthia McFadden and Sheila Nevins. Stritch has added 13 more dates at the Carlyle starting March 22.