Properly link flecs library

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

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+#include <monitor.h>
+#include <iostream>
+
+// A monitor observer triggers when an entity starts/stop matching the observer
+// filter. The observer communicates whether an entity is "entering/leaving" the
+// monitor by setting ecs_iter_t::event to EcsOnAdd (for entering) or 
+// EcsOnRemove (for leaving).
+//
+// To specify that an observer is a monitor observer, the EcsMonitor tag must be
+// provided as event. No additional event kinds should be provided for a monitor
+// observer.
+
+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::Monitor) // Monitor entities entering/leaving the query
+        .each([](flecs::iter& it, size_t i, Position&, Velocity&) {
+            if (it.event() == flecs::OnAdd) {
+                std::cout << " - Enter: "
+                    << it.event_id().str() << ": "
+                    << it.entity(i).name() << "\n";
+            } else if (it.event() == flecs::OnRemove) {
+                std::cout << " - Leave: "
+                    << it.event_id().str() << ": "
+                    << it.entity(i).name() << "\n";
+            }
+        });
+
+    // Create entity
+    flecs::entity e = ecs.entity("e");
+
+    // This does not yet trigger the monitor, as the entity does not yet match.
+    e.set<Position>({10, 20});
+
+    // This triggers the monitor with EcsOnAdd, as the entity now matches.
+    e.set<Velocity>({1, 2});
+
+    // This triggers the monitor with EcsOnRemove, as the entity no longer matches.
+    e.remove<Position>();
+
+    // Output
+    //   - Enter: Velocity: e
+    //   - Leave: Position: e
+}