Properly link flecs library

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

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+#include <custom_phases.h>
+#include <iostream>
+
+// 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 flecs::Phase tag.
+
+// Dummy system
+void Sys(flecs::iter& it) {
+    std::cout << "system " << it.system().name() << "\n";
+}
+
+int main(int argc, char *argv[]) {
+    flecs::world ecs(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.
+    flecs::entity Physics = ecs.entity()
+        .add(flecs::Phase)
+        .depends_on(flecs::OnUpdate);
+
+    flecs::entity Collisions = ecs.entity()
+        .add(flecs::Phase)
+        .depends_on(Physics);
+
+    // Create 3 dummy systems.
+    ecs.system("CollisionSystem")
+        .kind(Collisions)
+        .iter(Sys);
+
+    ecs.system("PhysicsSystem")
+        .kind(Physics)
+        .iter(Sys);
+
+    ecs.system("GameSystem")
+        .kind(flecs::OnUpdate)
+        .iter(Sys);
+
+    // Run pipeline
+    ecs.progress();
+
+    // Output
+    //   system GameSystem
+    //   system PhysicsSystem
+    //   system CollisionSystem
+}