Piecewise construct to avoid invalid construction

In CMainSignals::RegisterWithMempoolSignals running under Ubuntu 14.04
(QT 5.2), absent piecewise construction this fails to create the pair
because the argument is a connection, which is converted into a
non-copyable scoped_connection.

    validationinterface.cpp:80:186:   required from here
    /usr/include/boost/signals2/connection.hpp:234:7: error: ‘boost::signals2::scoped_connection::scoped_connection(const boost::signals2::scoped_connection&)’ is private
           scoped_connection(const scoped_connection &other);
           ^
    In file included from /usr/include/c++/4.8/utility:70:0,
                     from /usr/include/c++/4.8/algorithm:60,
                     from ./prevector.h:13,
                     from ./script/script.h:10,
                     from ./primitives/transaction.h:11,
                     from ./validationinterface.h:9,
                     from validationinterface.cpp:6:
    /usr/include/c++/4.8/bits/stl_pair.h:134:45: error: within this context
      : first(std::forward<_U1>(__x)), second(__y) { }
https://travis-ci.org/bitcoin/bitcoin/jobs/473689141#L2172

src/validationinterface.cpp

@@ -14,6 +14,7 @@
 #include <list>
 #include <atomic>
 #include <future>
+#include <utility>
 
 #include <boost/signals2/signal.hpp>
 
@@ -77,7 +78,10 @@ size_t CMainSignals::CallbacksPending() {
 }
 
 void CMainSignals::RegisterWithMempoolSignals(CTxMemPool& pool) {
-    g_connNotifyEntryRemoved.emplace(&pool, pool.NotifyEntryRemoved.connect(std::bind(&CMainSignals::MempoolEntryRemoved, this, std::placeholders::_1, std::placeholders::_2)));
+    g_connNotifyEntryRemoved.emplace(std::piecewise_construct,
+        std::forward_as_tuple(&pool),
+        std::forward_as_tuple(pool.NotifyEntryRemoved.connect(std::bind(&CMainSignals::MempoolEntryRemoved, this, std::placeholders::_1, std::placeholders::_2)))
+    );
 }
 
 void CMainSignals::UnregisterWithMempoolSignals(CTxMemPool& pool) {