Properly link flecs library

engine/libs/flecs/examples/cpp/queries/group_iter/src/main.cpp

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+#include <group_iter.h>
+#include <iostream>
+
+// A group iterator iterates over a single group of a grouped query (see the 
+// group_by example for more details). This can be useful when an application
+// may need to match different entities based on the context of the game, such 
+// as editor mode, day/night, inside/outside or location in the world.
+//
+// One example is that of an open game which is divided up into world 
+// cells. Even though a world may contain many entities, only the entities in 
+// cells close to the player need to be processed.
+//
+// Instead of creating a cached query per world cell, which could be expensive
+// as there are more caches to keep in sync, applications can create a single
+// query grouped by world cell, and use group iterators to only iterate the
+// necessary cells.
+
+// A world cell relationship with four cells
+struct WorldCell {};
+struct Cell_0_0 {};
+struct Cell_0_1 {};
+struct Cell_1_0 {};
+struct Cell_1_1 {};
+
+// Npc tags
+struct Npc {};
+struct Merchant {};
+struct Soldier {};
+struct Beggar {};
+struct Mage {};
+
+int main() {
+    flecs::world ecs;
+
+    // Create npc's in world cell 0_0
+    ecs.entity().add<WorldCell, Cell_0_0>()
+        .add<Merchant>()
+        .add<Npc>();
+    ecs.entity().add<WorldCell, Cell_0_0>()
+        .add<Merchant>()
+        .add<Npc>();
+
+    // Create npc's in world cell 0_1
+    ecs.entity().add<WorldCell, Cell_0_1>()
+        .add<Beggar>()
+        .add<Npc>();
+    ecs.entity().add<WorldCell, Cell_0_1>()
+        .add<Soldier>()
+        .add<Npc>();
+
+    // Create npc's in world cell 1_0
+    ecs.entity().add<WorldCell, Cell_1_0>()
+        .add<Mage>()
+        .add<Npc>();
+    ecs.entity().add<WorldCell, Cell_1_0>()
+        .add<Beggar>()
+        .add<Npc>();
+
+    // Create npc's in world cell 1_1
+    ecs.entity().add<WorldCell, Cell_1_1>()
+        .add<Soldier>()
+        .add<Npc>();
+
+    flecs::query<Npc> q = ecs.query_builder<Npc>()
+        .group_by<WorldCell>()
+        .build();
+
+    // Iterate all tables
+    std::cout << "All tables:\n";
+    q.iter([&](flecs::iter& it) {
+        flecs::entity group = ecs.entity(it.group_id());
+        std::cout << " - group " << group.path() << ": table ["
+            << it.table().str() << "]\n";
+    });
+    
+    std::cout << "\n";
+
+    // Only iterate entities in cell 1_0
+    std::cout << "Tables for cell 1_0:\n";
+    q.iter().set_group<Cell_1_0>().iter([&](flecs::iter& it) {
+        flecs::entity group = ecs.entity(it.group_id());
+        std::cout << " - group " << group.path() << ": table ["
+            << it.table().str() << "]\n";
+    });
+    
+    // Output:
+    //  All tables:
+    //   - group ::Cell_0_0: table [Merchant, Npc, (WorldCell,Cell_0_0)]
+    //   - group ::Cell_0_1: table [Npc, Beggar, (WorldCell,Cell_0_1)]
+    //   - group ::Cell_0_1: table [Npc, Soldier, (WorldCell,Cell_0_1)]
+    //   - group ::Cell_1_0: table [Npc, Mage, (WorldCell,Cell_1_0)]
+    //   - group ::Cell_1_0: table [Npc, Beggar, (WorldCell,Cell_1_0)]
+    //   - group ::Cell_1_1: table [Npc, Soldier, (WorldCell,Cell_1_1)]
+    //  
+    //  Tables for cell 1_0:
+    //   - group ::Cell_1_0: table [Npc, Mage, (WorldCell,Cell_1_0)]
+    //   - group ::Cell_1_0: table [Npc, Beggar, (WorldCell,Cell_1_0)]
+}