Properly link flecs library

engine/libs/flecs/examples/cpp/queries/singleton/src/main.cpp

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+#include <singleton.h>
+#include <iostream>
+
+// This example shows how to use singleton components in queries.
+
+// Singleton component
+struct Gravity {
+    double value; 
+};
+
+// Entity component
+struct Velocity {
+    double x;
+    double y;
+};
+
+int main(int, char *[]) {
+    flecs::world world;
+
+    // Set singleton
+    world.set<Gravity>({ 9.81 });
+
+    // Set Velocity
+    world.entity("e1").set<Velocity>({0, 0});
+    world.entity("e2").set<Velocity>({0, 1});
+    world.entity("e3").set<Velocity>({0, 2});
+
+    // Create query that matches Gravity as singleton
+    flecs::query<Velocity, const Gravity> q = 
+        world.query_builder<Velocity, const Gravity>()
+            .term_at(2).singleton()
+            .build();
+
+    // In a query string expression you can use the $ shortcut for singletons:
+    //   Velocity, Gravity($)
+
+    q.each([](flecs::entity e, Velocity& v, const Gravity& g) {
+        v.y += g.value;
+        std::cout << e.path() << " velocity is {" 
+            << v.x << ", " << v.y << "}" << std::endl;
+    });
+
+    // Output
+    //   ::e1 velocity is {0, 9.81}
+    //   ::e2 velocity is {0, 10.81}
+    //   ::e3 velocity is {0, 11.81}
+}