Convert uses of boost::flat_set to std::set

The performance difference is irrelevant with the small number of elements.

src/game_events/conditional_wml.cpp

@@ -169,7 +169,7 @@ namespace { // Support functions
 		}
 
 		vconfig::all_children_iterator cond_end = cond.ordered_end();
-		static const boost::container::flat_set<std::string> hard_coded =
+		static const std::set<std::string> hard_coded =
 			{"true", "false", "have_unit", "have_location", "variable", "then", "else", "elseif", "not", "and", "or", "do"};
 
 		assert(resources::lua_kernel);

src/play_controller.cpp

@@ -87,9 +87,7 @@ static lg::log_domain log_engine_enemies("engine/enemies");
  */
 static void copy_persistent(const config& src, config& dst)
 {
-	typedef boost::container::flat_set<std::string> stringset;
-
-	static stringset attrs = {
+	static const std::set<std::string> attrs = {
 			"id",
 			"theme",
 			"next_scenario",
@@ -103,7 +101,7 @@ static void copy_persistent(const config& src, config& dst)
 			"experience_modifier",
 			"require_scenario"};
 
-	static stringset tags = {
+	static const std::set<std::string> tags = {
 			"terrain_graphics",
 			"lua"};
 

src/team.cpp

@@ -54,7 +54,7 @@ const int team::default_team_gold_ = 100;
 
 // Update this list of attributes if you change what is used to define a side
 // (excluding those attributes used to define the side's leader).
-const boost::container::flat_set<std::string> team::attributes = {"ai_config",
+const std::set<std::string> team::attributes = {"ai_config",
 	"carryover_add", "carryover_percentage", "color", "controller", "current_player", "defeat_condition", "flag",
 	"flag_icon", "fog", "fog_data", "gold", "hidden", "income",
 	"no_leader", "objectives", "objectives_changed", "persistent", "lost",

src/team.hpp

@@ -23,7 +23,7 @@
 #include "util.hpp"
 #include "config.hpp"
 
-#include <boost/container/flat_set.hpp>
+#include <set>
 
 class game_data;
 class gamemap;
@@ -168,7 +168,7 @@ class team
 
 	/// Stores the attributes recognized by [side]. These should be stripped
 	/// from a side's config before using it to create the side's leader.
-	static const boost::container::flat_set<std::string> attributes;
+	static const std::set<std::string> attributes;
 
 	void build(const config &cfg, const gamemap &map, int gold = default_team_gold_);