Properly link flecs library

engine/libs/flecs/examples/c/rules/cyclic_variables/src/main.c

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+#include <cyclic_variables.h>
+#include <stdio.h>
+
+// This example shows how a rule may have terms with cyclic dependencies on
+// variables.
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_TAG(ecs, Likes); // Likes relationship
+
+    ecs_entity_t bob = ecs_new_entity(ecs, "Bob");
+    ecs_entity_t alice = ecs_new_entity(ecs, "Alice");
+    ecs_entity_t john = ecs_new_entity(ecs, "John");
+    ecs_entity_t jane = ecs_new_entity(ecs, "Jane");
+
+    ecs_add_pair(ecs, bob, Likes, alice);
+    ecs_add_pair(ecs, alice, Likes, bob);
+    ecs_add_pair(ecs, john, Likes, jane);
+    ecs_add_pair(ecs, jane, Likes, john);
+    ecs_add_pair(ecs, bob, Likes, jane); // inserting a bit of drama
+
+    // The following rule will only return entities that have a cyclic Likes
+    // relationship- that is they must both like each other.
+    //
+    // The equivalent query in the DSL is:
+    //   Likes($X, $Y), Likes($Y, $X)
+    //
+    // This is also an example of a query where all sources are variables. By
+    // default queries use the builtin "This" variable as subject, which is what
+    // populates the entities array in the query result (accessed by the
+    // iter::entity function).
+    //
+    // Because this query does not use This at all, the entities array will not
+    // be populated, and it.count() will always be 0.
+    // Create a rule to find all ranged units
+    ecs_rule_t *r = ecs_rule(ecs, {
+        .terms = {
+            { 
+                .first.id = Likes, 
+                .src = { .name = "X", .flags = EcsIsVariable },
+                .second = { .name = "Y", .flags = EcsIsVariable }
+            },
+            { 
+                .first.id = Likes, 
+                .src = { .name = "Y", .flags = EcsIsVariable },
+                .second = { .name = "X", .flags = EcsIsVariable }
+            }
+        }
+    });
+
+    // Lookup the index of the variables. This will let us quickly lookup their
+    // values while we're iterating.
+    int x_var = ecs_rule_find_var(r, "X");
+    int y_var = ecs_rule_find_var(r, "Y");
+
+    // Iterate the rule
+    ecs_iter_t it = ecs_rule_iter(ecs, r);
+    while (ecs_rule_next(&it)) {
+        ecs_entity_t x = ecs_iter_get_var(&it, x_var);
+        ecs_entity_t y = ecs_iter_get_var(&it, y_var);
+        printf("%s likes %s\n", ecs_get_name(ecs, x), ecs_get_name(ecs, y));
+    }
+
+    ecs_rule_fini(r);
+
+    // Output
+    //  Bob likes Alice
+    //  Jane likes John
+    //  John likes Jane
+    //  Alice likes Bob
+
+    // Note that the rule returns each pair twice. The reason for this is that
+    // the goal of the rule engine is to return all "facts" that are true
+    // within the given constraints. Since we did not give it any constraints
+    // that would favor a person being matched by X or Y, the rule engine
+    // returns both.
+
+    return ecs_fini(ecs);
+}