Build with newer Boost. From Fedora.

Index: gr-zeromq/lib/pub_msg_sink_impl.cc
--- gr-zeromq/lib/pub_msg_sink_impl.cc.orig
+++ gr-zeromq/lib/pub_msg_sink_impl.cc
@@ -31,6 +31,8 @@
 namespace gr {
 namespace zeromq {
 
+using namespace boost::placeholders;
+
 pub_msg_sink::sptr pub_msg_sink::make(char* address, int timeout, bool bind)
 {
     return gnuradio::get_initial_sptr(new pub_msg_sink_impl(address, timeout, bind));
