Properly link flecs library

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

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+#include <custom_phases.h>
+#include <stdio.h>
+
+// This application demonstrates how to use custom phases for systems. The 
+// default pipeline will automatically run systems for custom phases as long as 
+// they have the EcsPhase tag.
+
+// Dummy system
+void Sys(ecs_iter_t *it) {
+    printf("system %s\n", ecs_get_name(it->world, it->system));
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    // Create two custom phases that branch off of EcsOnUpdate. Note that the
+    // phases have the Phase tag, which is necessary for the builtin pipeline
+    // to discover which systems it should run.
+    ecs_entity_t Physics = ecs_new_w_id(ecs, EcsPhase);
+    ecs_entity_t Collisions = ecs_new_w_id(ecs, EcsPhase);
+    ecs_add_pair(ecs, Physics, EcsDependsOn, EcsOnUpdate);
+    ecs_add_pair(ecs, Collisions, EcsDependsOn, Physics);
+
+    // Create 3 dummy systems.
+    ecs_system(ecs, {
+        .entity = ecs_entity(ecs, {
+            .name = "CollisionSystem",
+            .add = { ecs_dependson(Collisions) }
+        }),
+        .callback = Sys
+    });
+
+    ecs_system(ecs, {
+        .entity = ecs_entity(ecs, {
+            .name = "PhysicsSystem",
+            .add = { ecs_dependson(Physics) }
+        }),
+        .callback = Sys
+    });
+
+    ecs_system(ecs, {
+        .entity = ecs_entity(ecs, {
+            .name = "GameSystem",
+            .add = { ecs_dependson(EcsOnUpdate) }
+        }),
+        .callback = Sys
+    });
+
+    // Run pipeline
+    ecs_progress(ecs, 0);
+
+    return ecs_fini(ecs);
+
+    // Output
+    //   system GameSystem
+    //   system PhysicsSystem
+    //   system CollisionSystem
+}