Properly link flecs library

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

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+#include <custom_pipeline.h>
+#include <iostream>
+
+// Custom pipelines make it possible for applications to override which systems
+// are ran by a pipeline and how they are ordered. Pipelines are queries under
+// the hood, and custom pipelines override the query used for system matching.
+
+// If you only want to use custom phases in addition or in place of the builtin
+// phases see the custom_phases and custom_phases_no_builtin examples, as this 
+// does not require using a custom pipeline.
+
+struct Physics {};
+
+int main(int argc, char *argv[]) {
+    flecs::world ecs(argc, argv);
+
+    // Create a pipeline that matches systems with Physics. Note that this
+    // pipeline does not require the use of phases (see custom_phases) or of the
+    // DependsOn relationship.
+    flecs::entity pipeline = ecs.pipeline()
+        .with(flecs::System) // Mandatory, must always match systems
+        .with<Physics>()
+        .build();
+
+    // Configure the world to use the custom pipeline
+    ecs.set_pipeline(pipeline);
+
+    // Create system with Physics tag
+    ecs.system()
+        .kind<Physics>()
+        .iter([](flecs::iter&) {
+            std::cout << "System ran!\n";
+        });
+
+    // Runs the pipeline & system
+    ecs.progress();
+
+    // Output:
+    //   System ran!
+}