Properly link flecs library

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

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+#include <exclusive_relations.h>
+#include <iostream>
+
+// Type for Platoon relationship
+struct Platoon { };
+
+int main(int, char*[]) {
+    flecs::world ecs;
+
+    // Register Platoon as exclusive relationship. This ensures that an entity
+    // can only belong to a single Platoon.
+    ecs.component<Platoon>()
+        .add(flecs::Exclusive);
+
+    // Create two platoons
+    flecs::entity platoon_1 = ecs.entity();
+    flecs::entity platoon_2 = ecs.entity();
+
+    // Create a unit
+    flecs::entity unit = ecs.entity();
+
+    // Add unit to platoon 1
+    unit.add<Platoon>(platoon_1);
+
+    // Log platoon of unit
+    std::cout << "Unit in platoon 1: " << 
+        (unit.has<Platoon>(platoon_1) ? "true" : "false") << "\n";
+    std::cout << "Unit in platoon 2: " << 
+        (unit.has<Platoon>(platoon_2) ? "true" : "false") << "\n\n";
+
+    // Add unit to platoon 2. Because Platoon is an exclusive relationship, this
+    // both removes (Platoon, platoon_1) and adds (Platoon, platoon_2) in a
+    // single operation.
+    unit.add<Platoon>(platoon_2);
+
+    std::cout << "Unit in platoon 1: " << 
+        (unit.has<Platoon>(platoon_1) ? "true" : "false") << "\n";
+    std::cout << "Unit in platoon 2: " << 
+        (unit.has<Platoon>(platoon_2) ? "true" : "false") << "\n";
+
+    // Output:
+    //  Unit in platoon 1: true
+    //  Unit in platoon 2: false
+    //  
+    //  Unit in platoon 1: false
+    //  Unit in platoon 2: true
+}