Properly link flecs library

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

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+#include <relation_component.h>
+#include <iostream>
+
+// This example shows how relationships can be combined with components to attach
+// data to a relationship.
+
+// Some demo components:
+
+struct Requires {
+  double amount;
+};
+
+struct Gigawatts { };
+
+struct Expires {
+    double timeout;
+};
+
+struct Position {
+    double x;
+    double y;
+};
+
+struct MustHave { };
+
+int main(int, char*[]) {
+    flecs::world ecs;
+
+    // When one element of a pair is a component and the other element is a tag, 
+    // the pair assumes the type of the component. 
+    flecs::entity e1 = ecs.entity().set<Requires, Gigawatts>({1.21});
+    const Requires *r = e1.get<Requires, Gigawatts>();
+    std::cout << "requires: " << r->amount << std::endl;
+
+    // The component can be either the first or second part of a pair:
+    flecs::entity e2 = ecs.entity().set<Gigawatts, Requires>({1.21});
+    r = e2.get<Gigawatts, Requires>();
+    std::cout << "requires: " << r->amount << std::endl;
+
+    // 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:
+    flecs::entity e3 = ecs.entity().set<Expires, Position>({0.5});
+    const Expires *e = e3.get<Expires, Position>();
+    std::cout << "expires: " << e->timeout << std::endl;
+
+    // You can prevent a pair from assuming the type of a component by adding
+    // the Tag property to a relationship:
+    ecs.component<MustHave>().add(flecs::Tag);
+
+    // Even though Position is a component, <MustHave, Position> contains no
+    // data because MustHave has the Tag property.
+    ecs.entity().add<MustHave, Position>();
+
+    // The id::type_id method can be used to find the component type for a pair:
+    std::cout << ecs.pair<Requires, Gigawatts>().type_id().path() << "\n";
+    std::cout << ecs.pair<Gigawatts, Requires>().type_id().path() << "\n";
+    std::cout << ecs.pair<Expires, Position>().type_id().path() << "\n";
+    std::cout << ecs.pair<MustHave, Position>().type_id().path() << "\n";
+
+    // When querying for a relationship component, add the pair type as template
+    // argument to the builder:
+    flecs::query<Requires> q = ecs.query_builder<Requires>()
+        .term_at(1).second<Gigawatts>() // set second part of pair for first term
+        .build();
+    
+    // When iterating, always use the pair type:
+    q.each([](Requires& rq) {
+        std::cout << "requires " << rq.amount << " gigawatts\n";
+    });
+
+    // Output:
+    //  requires: 1.21
+    //  requires: 1.21
+    //  expires: 0.5
+    //  ::Requires
+    //  ::Requires
+    //  ::Expires
+    //  0
+    //  requires 1.21 gigawatts
+}