Build with newer Boost. From Fedora.

Index: gr-blocks/lib/message_strobe_random_impl.cc
--- gr-blocks/lib/message_strobe_random_impl.cc.orig
+++ gr-blocks/lib/message_strobe_random_impl.cc
@@ -38,6 +38,8 @@
 namespace gr {
 namespace blocks {
 
+using namespace boost::placeholders;
+
 message_strobe_random::sptr
 message_strobe_random::make(pmt::pmt_t msg,
                             message_strobe_random_distribution_t dist,
