Build with newer Boost. From Fedora.

Index: gr-blocks/lib/multiply_matrix_impl.cc
--- gr-blocks/lib/multiply_matrix_impl.cc.orig
+++ gr-blocks/lib/multiply_matrix_impl.cc
@@ -31,6 +31,8 @@
 namespace gr {
 namespace blocks {
 
+using namespace boost::placeholders;
+
 // Copy tags from input k to output l if A[l][k] is not zero
 template <>
 void multiply_matrix_impl<gr_complex>::propagate_tags_by_A(int noutput_items,
