Properly link flecs library

engine/libs/flecs/test/custom_builds/c/system/src/main.c

@@ -0,0 +1,40 @@
+#include <system.h>
+
+typedef struct {
+    int32_t x, y;
+} Position, Velocity;
+
+void Move(ecs_iter_t *it) {
+    Position *p = ecs_field(it, Position, 1);
+    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;
+    }
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *world = ecs_init_w_args(argc, argv);
+
+    ECS_COMPONENT(world, Position);
+    ECS_COMPONENT(world, Velocity);
+
+    ecs_entity_t s = ecs_system_init(world, &(ecs_system_desc_t){
+        .query.filter.terms = {{ .id = ecs_id(Position) }, { .id = ecs_id(Velocity) }},
+        .callback = Move,
+    });
+    assert(s != 0);
+
+    ecs_entity_t e = ecs_new_id(world);
+    ecs_set(world, e, Position, {10, 20});
+    ecs_set(world, e, Velocity, {1, 2});
+
+    ecs_run(world, s, 0, 0);
+
+    const Position *p = ecs_get(world, e, Position);
+    assert(p->x == 11);
+    assert(p->y == 22);
+
+    return ecs_fini(world);
+}