Properly link flecs library

engine/libs/flecs/examples/cpp/observers/two_components/src/main.cpp

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+#include <two_components.h>
+#include <iostream>
+
+// An observer can match multiple components/tags. Only entities that match the
+// entire observer filter will be forwarded to the callback. For example, an
+// observer for Position,Velocity won't match an entity that only has Position.
+
+struct Position {
+    double x, y;
+};
+
+struct Velocity {
+    double x, y;
+};
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    // Create observer for custom event
+    ecs.observer<Position, Velocity>()
+        .event(flecs::OnSet)
+        .each([](flecs::iter& it, size_t i, Position& p, Velocity& v) {
+            std::cout << " - " << it.event().name() << ": " 
+                << it.event_id().str() << ": "
+                << it.entity(i).name() << ": "
+                << "p: {" << p.x << ", " << p.y << "} "
+                << "v: {" << v.x << ", " << v.y << "}\n";
+        });
+
+    // Create entity, set Position (emits EcsOnSet, does not yet match observer)
+    flecs::entity e = ecs.entity("e")
+        .set<Position>({10, 20});
+
+    // Set Velocity (emits EcsOnSet, matches observer)
+    e.set<Velocity>({1, 2});
+
+    // Output
+    //   - OnSet: Velocity: e: p: {10, 20} v: {1, 2}
+}