Build with newer Boost. From Fedora.

Index: gr-fec/lib/async_encoder_impl.cc
--- gr-fec/lib/async_encoder_impl.cc.orig
+++ gr-fec/lib/async_encoder_impl.cc
@@ -32,6 +32,8 @@
 namespace gr {
 namespace fec {
 
+using namespace boost::placeholders;
+
 async_encoder::sptr async_encoder::make(generic_encoder::sptr my_encoder,
                                         bool packed,
                                         bool rev_unpack,
