Build with newer Boost. From Fedora.

Index: gr-blocks/lib/random_pdu_impl.cc
--- gr-blocks/lib/random_pdu_impl.cc.orig
+++ gr-blocks/lib/random_pdu_impl.cc
@@ -31,6 +31,8 @@
 namespace gr {
 namespace blocks {
 
+using namespace boost::placeholders;
+
 random_pdu::sptr
 random_pdu::make(int min_items, int max_items, unsigned char byte_mask, int length_modulo)
 {
