Properly link flecs library

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

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+#include <iostream>
+#include <entity_event.h>
+
+// Entity events are events that are emitted and observed for a specific entity.
+// They are a thin wrapper around regular observers, which match against queries
+// instead of single entities. While they work similarly under the hood, entity
+// events provide a much simpler API.
+//
+// An entity event only needs two pieces of data:
+// - The entity on which to emit the event
+// - The event to emit
+
+// An event without payload
+struct Click { };
+
+// An event with payload
+struct Resize {
+    double width, height;
+};
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    // Create a widget entity
+    flecs::entity widget = ecs.entity("MyWidget");
+
+    // Observe the Click event on the widget entity.
+    widget.observe<Click>([]() {
+        std::cout << "clicked!\n";
+    });
+
+    // Observers can have an entity argument that holds the event source.
+    // This allows the same function to be reused for different entities.
+    widget.observe<Click>([](flecs::entity src) {
+        std::cout << "clicked on " << src.path() << "!\n";
+    });
+
+    // Observe the Resize event. Events with payload are passed as arguments
+    // to the observer callback.
+    widget.observe([](Resize& p) {
+        std::cout << "resized to {" 
+                  << p.width << ", " << p.height << "}!\n"; 
+    });
+
+    // Emit the Click event
+    widget.emit<Click>();
+
+    // Emit the Resize event
+    widget.emit<Resize>({100, 200});
+
+    // Output
+    //   clicked!
+    //   clicked on ::MyWidget!
+    //   resized to {100, 200}!
+}