Make listener<>::instances inline

This simplifies the user interface and removes a clang warning about it being
potentially uninitialized.

See #2139.

client/chatline.cpp

@@ -43,7 +43,6 @@
 
 static bool is_plain_public_message(const QString &s);
 
-FC_CPP_DECLARE_LISTENER(chat_listener)
 QStringList chat_listener::history = QStringList();
 
 namespace {

client/listener.h

@@ -27,13 +27,6 @@
   };
   ~~~~~
 
-  The listener needs some static data. Declaring it is as simple as putting
-  a macro in some source file:
-
-  ~~~~~{.cpp}
-  FC_CPP_DECLARE_LISTENER(foo_listener)
-  ~~~~~
-
   Then, you call the listeners from the implementation of the C interface:
 
   ~~~~~{.cpp}
@@ -104,7 +97,7 @@ template <class _type_> class listener {
 
 private:
   // All instances of type_t that have called listen().
-  static std::set<listener<type_t> *> instances;
+  inline static std::set<listener<type_t> *> instances{};
 
 protected:
   explicit listener();
@@ -117,14 +110,6 @@ template <class _type_> class listener {
   static void invoke(_member_fct_ function, _args_ &&...args);
 };
 
-/***************************************************************************
-  Macro to declare the static data needed by listener<> classes
-***************************************************************************/
-#define FC_CPP_DECLARE_LISTENER(_type_)                                     \
-  template <>                                                               \
-  std::set<listener<_type_> *> listener<_type_>::instances =                \
-      std::set<listener<_type_> *>();
-
 /***************************************************************************
   Constructor
 ***************************************************************************/

client/map_updates_handler.cpp

@@ -3,8 +3,6 @@
 #include "options.h"
 #include "tilespec.h"
 
-FC_CPP_DECLARE_LISTENER(freeciv::map_updates_handler)
-
 namespace freeciv {
 
 /**