(PR#10145) Inefficiency in adjust_building_want_by_effects

<URL: http://rt.freeciv.org/Ticket/Display.html?id=10145 >

  cities[EFR_CONTINENT] = 0;
  city_list_iterate(pplayer->cities, acity) {
    if (map_get_tile(acity->x, acity->y)->continent == ptile->continent)
      cities[EFR_CONTINENT]++;
  } city_list_iterate_end;

We can take the value of cites[EFR_CONTINENT] from AI statistics in O(1)
time.
--
mateusz