Cast boost::shared_ptr explicitly for C++14.
See also: https://bugs.gentoo.org/show_bug.cgi?id=594674

--- a/src/controller_slot.cpp
+++ b/src/controller_slot.cpp
@@ -67,9 +67,9 @@ ControllerSlot::disconnect()
 }
 
 bool
 ControllerSlot::is_connected() const
 {
-  return m_thread;
+  return static_cast<bool>(m_thread);
 }
 
 /* EOF */