Properly link flecs library

engine/libs/flecs/examples/c/relationships/relation_component/src/main.c

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+#include <relation_component.h>
+#include <stdio.h>
+
+// This example shows how relationships can be combined with components to attach
+// data to a relationship.
+
+// Some demo components:
+
+typedef struct {
+  double amount;
+} Requires;
+
+typedef struct {
+    double timeout;
+} Expires;
+
+typedef struct {
+    double x;
+    double y;
+} Position;
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_COMPONENT(ecs, Requires);
+    ECS_COMPONENT(ecs, Expires);
+    ECS_COMPONENT(ecs, Position);
+
+    ECS_TAG(ecs, Gigawatts);
+    ECS_TAG(ecs, MustHave);
+
+    // When one element of a pair is a component and the other element is a tag, 
+    // the pair assumes the type of the component. 
+    ecs_entity_t e1 = ecs_new_id(ecs);
+    ecs_set_pair(ecs, e1, Requires, Gigawatts, {1.21});
+    const Requires *r = ecs_get_pair(ecs, e1, Requires, Gigawatts);
+    printf("requires: %.2f\n", r->amount);
+
+    // The component can be either the first or second part of a pair:
+    ecs_entity_t e2 = ecs_new_id(ecs);
+    ecs_set_pair_second(ecs, e2, Gigawatts, Requires, {1.21});
+    r = ecs_get_pair_second(ecs, e2, Gigawatts, Requires);
+    printf("requires: %.2f\n", r->amount);
+
+    // Note that <Requires, Gigawatts> and <Gigawatts, Requires> are two 
+    // different pairs, and can be added to an entity at the same time.
+
+    // If both parts of a pair are components, the pair assumes the type of
+    // the first element:
+    ecs_entity_t e3 = ecs_new_id(ecs);
+    ecs_set_pair(ecs, e3, Expires, ecs_id(Position), {0.5});
+    const Expires *e = ecs_get_pair(ecs, e3, Expires, ecs_id(Position));
+    printf("expires: %.1f\n", e->timeout);
+
+    // You can prevent a pair from assuming the type of a component by adding
+    // the Tag property to a relationship:
+    ecs_add_id(ecs, MustHave, EcsTag);
+
+    // Even though Position is a component, <MustHave, Position> contains no
+    // data because MustHave has the Tag property.
+    ecs_new_w_pair(ecs, MustHave, ecs_id(Position));
+
+    // The id::type_id method can be used to find the component type for a pair:
+    ecs_entity_t type_id;
+    char *type_str;
+    
+    type_id = ecs_get_typeid(ecs, ecs_pair(ecs_id(Requires), Gigawatts));
+    type_str = ecs_get_fullpath(ecs, type_id);
+    printf("%s\n", type_str);
+    ecs_os_free(type_str);
+
+    type_id = ecs_get_typeid(ecs, ecs_pair(Gigawatts, ecs_id(Requires)));
+    type_str = ecs_get_fullpath(ecs, type_id);
+    printf("%s\n", type_str);
+    ecs_os_free(type_str);
+
+    type_id = ecs_get_typeid(ecs, ecs_pair(ecs_id(Expires), ecs_id(Position)));
+    type_str = ecs_get_fullpath(ecs, type_id);
+    printf("%s\n", type_str);
+    ecs_os_free(type_str);
+
+    type_id = ecs_get_typeid(ecs, ecs_pair(MustHave, ecs_id(Position)));
+    type_str = ecs_get_fullpath(ecs, type_id);
+    printf("%s\n", type_str);
+    ecs_os_free(type_str);
+
+    // When querying for a relationship, provide both parts of the pair:
+    ecs_query_t *q = ecs_query(ecs, {
+        .filter.terms = {
+            { .id = ecs_pair(ecs_id(Requires), Gigawatts) }
+        }
+    });
+
+    // When iterating, always use the pair type:
+    ecs_iter_t it = ecs_query_iter(ecs, q);
+    while (ecs_query_next(&it)) {
+        r = ecs_field(&it, Requires, 1);
+        for (int i = 0; i < it.count; i ++) {
+            printf("requires %.2f gigawatts\n", r[i].amount);
+        }
+    }
+
+    // Output:
+    //  requires: 1.21
+    //  requires: 1.21
+    //  expires: 0.5
+    //  Requires
+    //  Requires
+    //  Expires
+    //  0
+    //  requires 1.21 gigawatts
+}