cmake: include libdl in link only if Legion_USE_LIBDL=ON
cmake: include libdl in link only if Legion_USE_LIBDL=ON

StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion