Properly link flecs library

engine/libs/flecs/examples/c/systems/target_fps/src/main.c

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+#include <target_fps.h>
+#include <stdio.h>
+
+void PrintDeltaTime(ecs_iter_t *it) {
+    // Print delta_time. The same value is passed to all systems.
+    printf("delta_time: %f\n", (double)it->delta_time);
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    // Create system that prints delta_time. This system doesn't query for any
+    // components which means it won't match any entities, but will still be ran
+    // once for each call to ecs_progress.
+    ECS_SYSTEM(ecs, PrintDeltaTime, EcsOnUpdate, 0);
+
+    // Set target FPS to 1 frame per second
+    ecs_set_target_fps(ecs, 1);
+
+    // Run 5 frames
+    for (int i = 0; i < 5; i ++) {
+        // To make sure the frame doesn't run faster than the specified target
+        // FPS ecs_progress will insert a sleep if the measured delta_time is
+        // smaller than 1 / target_fps.
+        //
+        // By default ecs_progress uses the sleep function provided by the OS
+        // which is not always very accurate. If more accuracy is required the
+        // sleep function of the OS API can be overridden with a custom one.
+        //
+        // If a value other than 0 is provided to the delta_time argument of
+        // ecs_progress, this value will be used to calculate the length of
+        // the sleep to insert.
+        ecs_progress(ecs, 0);
+    }
+
+    return ecs_fini(ecs);
+}