cmake CPPZMQ_HAS_SHUTDOWN examples

gnuradio/gnuradio cmake/Modules/FindZeroMQ.cmake :29

check_cxx_source_compiles("
#include <zmq.hpp>
using _ = decltype(zmq::context_t().shutdown());
int main() { return 0; }
"
      CPPZMQ_HAS_SHUTDOWN
    )