cmake DDSRT_HAVE_MCAST_JOIN_SOURCE_GROUP examples

eclipse-cyclonedds/cyclonedds src/ddsrt/CMakeLists.txt :204 

check_symbol_exists("MCAST_JOIN_SOURCE_GROUP" ${sockopt_header} DDSRT_HAVE_MCAST_JOIN_SOURCE_GROUP)

eclipse-cyclonedds/cyclonedds src/ddsrt/CMakeLists.txt :215 

elseif(DDSRT_HAVE_IP_ADD_SOURCE_MEMBERSHIP AND DDSRT_HAVE_MCAST_JOIN_SOURCE_GROUP)