Properly link flecs library

engine/libs/flecs/examples/cpp/relationships/union/src/main.cpp

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+#include <union.h>
+#include <iostream>
+
+// This example shows how to use union relationships. Union relationships behave
+// much like exclusive relationships in that entities can have only one instance
+// and that adding an instance removes the previous instance.
+//
+// What makes union relationships stand out is that changing the relationship
+// target doesn't change the archetype of an entity. This allows for quick
+// switching of tags, which can be useful when encoding state machines in ECS.
+//
+// There is a tradeoff, and that is that because a single archetype can contain
+// entities with multiple targets, queries need to do a bit of extra work to
+// only return the requested target. 
+//
+// This code uses enumeration relationships. See the enum_relations example for
+// more details.
+
+enum Movement {
+    Walking,
+    Running
+};
+
+enum Direction {
+    Front,
+    Back,
+    Left,
+    Right
+};
+
+int main(int argc, char *argv[]) {
+    flecs::world ecs(argc, argv);
+
+    ecs.component<Movement>().add(flecs::Union);
+    ecs.component<Direction>().add(flecs::Union);
+
+    // Create a query that subscribes for all entities that have a Direction
+    // and that are walking
+    flecs::query<> q = ecs.query_builder()
+        .with(Walking)
+        .with<Direction>(flecs::Wildcard)
+        .build();
+
+    // Create a few entities with various state combinations
+    ecs.entity("e1")
+        .add(Walking)
+        .add(Front);
+
+    ecs.entity("e2")
+        .add(Running)
+        .add(Left);
+
+    flecs::entity e3 = ecs.entity("e3")
+        .add(Running)
+        .add(Back);
+
+    // Add Walking to e3. This will remove the Running case
+    e3.add(Walking);
+
+    // Iterate the query
+    q.iter([&](const flecs::iter& it) {
+        // Get the column with direction states. This is stored as an array
+        // with identifiers to the individual states
+        auto movement = it.field<flecs::entity_t>(1);
+        auto direction = it.field<flecs::entity_t>(2);
+
+        for (auto i : it) {
+            // Movement will always be Walking, Direction can be any state
+            std::cout << it.entity(i).name() 
+                << ": Movement: " 
+                << it.world().get_alive(movement[i]).name()
+                << ", Direction: "
+                << it.world().get_alive(direction[i]).name()
+                << std::endl;
+        }
+    });
+
+    // Output:
+    //   e3: Movement: Walking, Direction: Back
+    //   e1: Movement: Walking, Direction: Front
+
+    return 0;
+}