Properly link flecs library

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

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+#include <custom_pipeline.h>
+#include <stdio.h>
+
+// 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.
+
+// Dummy system
+void Sys(ecs_iter_t *it) {
+    (void)it;
+
+    printf("System ran!\n");
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_TAG(ecs, Physics);
+
+    // 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. 
+    ecs_entity_t pipeline = ecs_pipeline(ecs, {
+        .query.filter.terms = {
+            { .id = EcsSystem }, // Mandatory, must always match systems
+            { .id = Physics }
+        }
+    });
+
+    // Configure the world to use the custom pipeline
+    ecs_set_pipeline(ecs, pipeline);
+
+    // Create system with Physics tag
+    ECS_SYSTEM(ecs, Sys, Physics, 0);
+
+    // Runs the pipeline & system
+    ecs_progress(ecs, 0);
+
+    return ecs_fini(ecs);
+
+    // Output
+    //  System ran!
+}