Clark Gable is TCM’s Star of the Month – and tonight, in prime time, TCM presents a movie which NBC showed in two parts, in 1976, and got ratings strong enough to land in the all-time Top 10 at that point in television history. It’s 1939’s Gone with the Wind, and any time it’s shown in its proper screen ratio, unedited and uninterrupted, as it is tonight on TCM, you ought to watch it. And if you have what you think is a good excuse not to, well, frankly, my dear…