/home/lordaro/dev/openttd/src/core/smallstack_type.hpp:221:27: warning: instantiation of
      variable 'SmallStack<unsigned short, unsigned short, 65535, 8, 65533>::_pool' required
      here, but no definition is available [-Wundefined-var-template]
                        ThreadMutexLocker lock(_pool.GetMutex());
                                               ^
/home/lordaro/dev/openttd/src/cargopacket.h:502:32: note: in instantiation of member function
      'SmallStack<unsigned short, unsigned short, 65535, 8, 65533>::Pop' requested here
                        if (this->packets.find(next.Pop()) != this->packets.end()) return true;
                                                    ^
/home/lordaro/dev/openttd/src/core/smallstack_type.hpp:272:24: note: forward declaration of
      template entity is here
        static SmallStackPool _pool;
                              ^
/home/lordaro/dev/openttd/src/core/smallstack_type.hpp:221:27: note: add an explicit
      instantiation declaration to suppress this warning if 'SmallStack<unsigned short,
      unsigned short, 65535, 8, 65533>::_pool' is explicitly instantiated in another
      translation unit
                        ThreadMutexLocker lock(_pool.GetMutex());
                                               ^
1 warning generated.