Properly link flecs library

engine/libs/flecs/examples/cpp/entities/hierarchy/src/main.cpp

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+#include <hierarchy.h>
+#include <iostream>
+
+struct Position {
+    double x, y;
+};
+
+struct Star { };
+struct Planet { };
+struct Moon { };
+
+void iterate_tree(flecs::entity e, Position p_parent = {0, 0}) {
+    // Print hierarchical name of entity & the entity type
+    std::cout << e.path() << " [" << e.type().str() << "]\n";
+
+    // Get entity position
+    const Position *p = e.get<Position>();
+
+    // Calculate actual position
+    Position p_actual = {p->x + p_parent.x, p->y + p_parent.y};
+    std::cout << "{" << p_actual.x << ", " << p_actual.y << "}\n\n";
+
+    // Iterate children recursively
+    e.children([&](flecs::entity child) {
+        iterate_tree(child, p_actual);
+    });
+}
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    // Create a simple hierarchy.
+    // Hierarchies use ECS relationships and the builtin flecs::ChildOf relationship to
+    // create entities as children of other entities.
+
+    flecs::entity sun = ecs.entity("Sun")
+        .add<Star>()
+        .set<Position>({1, 1});
+
+        ecs.entity("Mercury")
+            .child_of(sun) // Shortcut for add(flecs::ChildOf, sun)
+            .add<Planet>()
+            .set<Position>({1, 1});
+
+        ecs.entity("Venus")
+            .child_of(sun)
+            .add<Planet>()
+            .set<Position>({2, 2});
+
+        flecs::entity earth = ecs.entity("Earth")
+            .child_of(sun)
+            .add<Planet>()
+            .set<Position>({3, 3});
+
+            flecs::entity moon = ecs.entity("Moon")
+                .child_of(earth)
+                .add<Moon>()
+                .set<Position>({0.1, 0.1});
+
+    // Is the Moon a child of Earth?
+    std::cout << "Child of Earth? " << moon.has(flecs::ChildOf, earth) << "\n\n";
+
+    // Do a depth-first walk of the tree
+    iterate_tree(sun);
+}