Build with newer Boost. From Fedora.

Index: gr-blocks/lib/tagged_stream_multiply_length_impl.cc
--- gr-blocks/lib/tagged_stream_multiply_length_impl.cc.orig
+++ gr-blocks/lib/tagged_stream_multiply_length_impl.cc
@@ -30,6 +30,8 @@
 namespace gr {
 namespace blocks {
 
+using namespace boost::placeholders;
+
 tagged_stream_multiply_length::sptr tagged_stream_multiply_length::make(
     size_t itemsize, const std::string& lengthtagname, double scalar)
 {
