Release Candidate v3.10.8.0-rc2 https://github.com/gnuradio/gnuradio/releases/tag/v3.10.8.0-rc2 detects the shutdown() function in zmq.hpp (from cppzmq or bundled with libzmq3-devel, typically). If that function is not found, gr-zeromq is disabled. RHEL8 EPEL has an old version of cppzmq. Rather than have compilation fail on that platform, we added this check.
