Properly link flecs library

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

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+#include <startup_system.h>
+#include <iostream>
+
+// Startup systems are systems registered with the EcsOnStart phase, and are
+// only ran during the first frame. Just like with regular phases, custom phases
+// can depend on the EcsOnStart phase (see custom_phases example). Phases that 
+// depend on EcsOnStart are also only ran during the first frame.
+//
+// Other than that, startup systems behave just like regular systems (they can
+// match components, can introduce merge points), with as only exception that
+// they are guaranteed to always run on the main thread. 
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    // Startup system
+    ecs.system("Startup")
+        .kind(flecs::OnStart)
+        .iter([](flecs::iter& it) {
+            std::cout << it.system().name() << "\n";
+        });
+
+    // Regular system
+    ecs.system("Update")
+        .iter([](flecs::iter& it) {
+            std::cout << it.system().name() << "\n";
+        });
+
+    // First frame. This runs both the Startup and Update systems
+    ecs.progress();
+
+    // Second frame. This runs only the Update system
+    ecs.progress();
+
+    // Output
+    //   Startup
+    //   Update
+    //   Update
+}