Properly link flecs library

engine/libs/flecs/examples/cpp/rules/basics/src/main.cpp

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+#include <basics.h>
+#include <iostream>
+
+struct Eats { };
+struct Healthy { };
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    flecs::entity Apples = ecs.entity("Apples").add<Healthy>();
+    flecs::entity Salad = ecs.entity("Salad").add<Healthy>();
+    flecs::entity Burgers = ecs.entity("Burgers");
+    flecs::entity Pizza = ecs.entity("Pizza");
+    flecs::entity Chocolate = ecs.entity("Chocolate");
+
+    ecs.entity("Bob")
+        .add<Eats>(Apples)
+        .add<Eats>(Burgers)
+        .add<Eats>(Pizza);
+
+    ecs.entity("Alice")
+        .add<Eats>(Salad)
+        .add<Eats>(Chocolate)
+        .add<Eats>(Apples);
+
+    // Here we're creating a rule that in the query DSL would look like this:
+    //   Eats($This, $Food), Healthy($Food)
+    //
+    // Rules are similar to queries, but support more advanced features. This
+    // example shows how the basics of how to use rules & variables.
+    flecs::rule<> r = ecs.rule_builder()
+        // Identifiers that start with _ are query variables. Query variables
+        // are like wildcards, but enforce that the entity substituted by the
+        // wildcard is the same across terms.
+        //
+        // For example, in this query it is not guaranteed that the entity
+        // substituted by the * for Eats is the same as for Healthy:
+        //   (Eats, *), Healthy(*)
+        //
+        // By replacing * with _Food, both terms are constrained to use the
+        // same entity.
+        .with<Eats>("$Food")
+        .with<Healthy>().src("$Food")
+        .build();
+
+    // Lookup the index of the variable. This will let us quickly lookup its
+    // value while we're iterating.
+    int food_var = r.find_var("Food");
+
+    // Iterate the rule
+    r.each([&](flecs::iter& it, size_t index) {
+        std::cout 
+            << it.entity(index).name()
+            << " eats " << it.get_var(food_var).name()
+            << "\n";
+    });
+
+    // Rules need to be explicitly deleted.
+    r.destruct();
+
+    // Output:
+    //  Bob eats Apples
+    //  Alice eats Apples
+    //  Alice eats Salad
+}