Properly link flecs library

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

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+#include <custom_runner.h>
+#include <stdio.h>
+
+// Systems can be created with a custom run function that takes control over the
+// entire iteration. By default, a system is invoked once per matched table, 
+// which means the function can be called multiple times per frame. In some
+// cases that's inconvenient, like when a system has things it needs to do only
+// once per frame. For these use cases, the run callback can be used which is
+// called once per frame per system.
+
+typedef struct {
+    double x, y;
+} Position, Velocity;
+
+void Move(ecs_iter_t *it) {
+    printf("Move begin\n");
+
+    // In a run function we have to manually iterate through the query results.
+    // Note that this code looks the same as iterating a query.
+    while (ecs_query_next(it)) {
+        // Inside the loop the code looks the same as for a regular system
+        Position *p = ecs_field(it, Position, 1);
+        const Velocity *v = ecs_field(it, Velocity, 2);
+
+        for (int i = 0; i < it->count; i ++) {
+            p[i].x += v[i].x;
+            p[i].y += v[i].y;
+            printf("%s: {%f, %f}\n", ecs_get_name(it->world, it->entities[i]), 
+                p[i].x, p[i].y);
+        }
+    }
+
+    printf("Move end\n");
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_COMPONENT(ecs, Position);
+    ECS_COMPONENT(ecs, Velocity);
+
+    // Create a system for Position, Velocity
+    ecs_entity_t move = ecs_system(ecs, {
+        .query.filter.terms = {
+            { .id = ecs_id(Position) },
+            { .id = ecs_id(Velocity), .inout = EcsIn }
+        },
+        .run = Move // use .run instead of .callback
+    });
+
+    // C applications can also use the ECS_SYSTEM convenience macro:
+    // ECS_SYSTEM(ecs, Move, 0, Position, [in] Velocity);
+
+    // Create a few test entities for a Position, Velocity query
+    ecs_entity_t e1 = ecs_new_entity(ecs, "e1");
+    ecs_set(ecs, e1, Position, {10, 20});
+    ecs_set(ecs, e1, Velocity, {1, 2});
+
+    ecs_entity_t e2 = ecs_new_entity(ecs, "e2");
+    ecs_set(ecs, e2, Position, {10, 20});
+    ecs_set(ecs, e2, Velocity, {3, 4});
+
+    // This entity will not match as it does not have Position, Velocity
+    ecs_entity_t e3 = ecs_new_entity(ecs, "e3");
+    ecs_set(ecs, e3, Position, {10, 20});
+
+    // Run the system
+    ecs_run(ecs, move, 0.0f, NULL);
+
+    return ecs_fini(ecs);
+
+    // Output
+    //  Move begin
+    //  e1: {11.000000, 22.000000}
+    //  e2: {13.000000, 24.000000}
+    //  Move end
+}