With a gala-style potpourri, the NDR Sinfonieorchester marked the 80th birthday of its principal conductor in his last season, Christoph von Dohnányi. The mix of repertoire, however, reflected the conductor’s wide-ranging, 20th-century-inclined enthusiasms. A eulogy was given by Alexander Pereira, elective chief of the Salzburg Festival, who praised his commitment to modernists such as Schoenberg and Ligeti while cultivating the classics. Dohnányi accompanied colleagues of long standing in virtuoso solo-spots by Ravel and Richard Strauss. Finally the orchestra’s ranks were swelled by members of three local youth orchestras for an ebullient Dvořák encore.