Properly link flecs library

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

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+#include <custom_event.h>
+#include <iostream>
+
+// Observers can be used to match custom events. Custom events can be emitted
+// using the ecs_emit function. This function is also used by builtin events,
+// so builtin and custom events use the same rules for matching with observers.
+//
+// An event consists out of three pieces of data used to match with observers:
+//  - An single event kind (EcsOnAdd, EcsOnRemove, ...)
+//  - One or more event ids (Position, Velocity, ...)
+//  - A source (either an entity or a table)
+
+struct Position {
+    double x, y;
+};
+
+// Create tag type to use as event (could also use entity)
+struct MyEvent { };
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    // Create observer for custom event
+    ecs.observer<Position>()
+        .event<MyEvent>()
+        .each([](flecs::iter& it, size_t i, Position&) {
+            std::cout << " - " << it.event().name() << ": " 
+                << it.event_id().str() << ": "
+                << it.entity(i).name() << "\n";
+        });
+
+    // The observer filter can be matched against the entity, so make sure it
+    // has the Position component before emitting the event. This does not 
+    // trigger the observer yet.
+    flecs::entity e = ecs.entity("e")
+        .set<Position>({10, 20});
+
+    // Emit the custom event
+    ecs.event<MyEvent>()
+        .id<Position>()
+        .entity(e)
+        .emit();
+
+    // Output
+    //   - MyEvent: Position: e
+}