Build with newer Boost. From Fedora.

Index: gr-blocks/lib/message_debug_impl.cc
--- gr-blocks/lib/message_debug_impl.cc.orig
+++ gr-blocks/lib/message_debug_impl.cc
@@ -32,6 +32,8 @@
 namespace gr {
 namespace blocks {
 
+using namespace boost::placeholders;
+
 message_debug::sptr message_debug::make()
 {
     return gnuradio::get_initial_sptr(new message_debug_impl());
