Properly link flecs library

engine/libs/flecs/examples/c/observers/yield_existing/src/main.c

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+#include <yield_existing.h>
+#include <stdio.h>
+
+// Observers can enable a "yield_existing" feature that upon creation of the
+// observer produces events for all entities that match the observer query. The
+// feature is only implemented for the builtin EcsOnAdd and EcsOnSet events.
+//
+// Custom events can also implement behavior for yield_existing by adding the
+// Iterable component to the event (see EcsIterable for more details).
+
+typedef struct {
+    double x, y;
+} Position;
+
+void Observer(ecs_iter_t *it) {
+    ecs_world_t *ecs = it->world;
+
+    ecs_entity_t event = it->event;
+    ecs_entity_t event_id = it->event_id;
+    Position *p = ecs_field(it, Position, 1);
+
+    for (int i = 0; i < it->count; i ++) {
+        ecs_entity_t e = it->entities[i];
+        printf(" - %s: %s: %s: {%f, %f}\n", 
+            ecs_get_name(ecs, event), 
+            ecs_get_name(ecs, event_id),
+            ecs_get_name(ecs, e),
+            p[i].x, p[i].y);
+    }
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_COMPONENT(ecs, Position);
+
+    // Create existing entities with Position component
+    ecs_entity_t e1 = ecs_new_entity(ecs, "e1");
+    ecs_set(ecs, e1, Position, {10, 20});
+    ecs_entity_t e2 = ecs_new_entity(ecs, "e2");
+    ecs_set(ecs, e2, Position, {20, 30});
+
+    ecs_observer(ecs, {
+        .filter = { .terms = {{ .id = ecs_id(Position) }}},
+        .events = { EcsOnSet },
+        .callback = Observer,
+        .yield_existing = true // Trigger for existing matching entities
+    });
+
+    ecs_fini(ecs);
+
+    // Output
+    //   - OnSet: Position: e1: {10.000000, 20.000000}
+    //   - OnSet: Position: e2: {20.000000, 30.000000}
+}