In a 1976 profile, the New York Times referred to Billy Dee Williams, the dashing leading man opposite Diana Ross in “Lady Sings the Blues” and “Mahogany,” as “the black Clark Gable.” But frankly, my dears, he did not give a damn. In his new memoir, “What Have We Here? Portraits of a Life,” he writes, “I wanted to be known as one of the best actors of my generation, period. But the opportunities weren’t the same for me as they were for Gable.”