Properly link flecs library

engine/libs/flecs/examples/cpp/systems/system_ctx/src/main.cpp

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+#include <system_ctx.h>
+#include <iostream>
+
+// Applications can pass context data to a system. A common use case where this
+// comes in handy is when a system needs to iterate more than one query. The
+// following example shows how to pass a custom query into a system for a simple
+// collision detection example.
+
+struct Position {
+    double x, y;
+};
+
+struct Radius {
+    double value;
+};
+
+double sqr(double value) {
+    return value * value;
+}
+
+double distance_sqr(const Position& p1, const Position& p2) {
+    return sqr(p2.x - p1.x) + sqr(p2.y - p1.y);
+}
+
+double randd(int max) {
+    return static_cast<double>((rand() % max));
+}
+
+using CollisionQuery = flecs::query<const Position, const Radius>;
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    CollisionQuery q_collide = ecs.query<const Position, const Radius>();
+
+    flecs::system sys = ecs.system<const Position, const Radius>("Collide")
+        .ctx(&q_collide)
+        .each([](flecs::iter& it, size_t i, const Position& p1, const Radius& r1) {
+            CollisionQuery *q = it.ctx<CollisionQuery>();
+            flecs::entity e1 = it.entity(i);
+
+            q->each([&](flecs::entity e2, const Position& p2, const Radius& r2) {
+                if (e1 == e2) {
+                    // don't collide with self
+                    return;
+                }
+
+                if (e1 > e2) {
+                    // Simple trick to prevent collisions from being detected
+                    // twice with the entities reversed.
+                    return;
+                }
+
+                // Check for collision
+                double d_sqr = distance_sqr(p1, p2);
+                double r_sqr = sqr(r1.value + r2.value);
+                if (r_sqr > d_sqr) {
+                    std::cout << e1 << " and " << e2 << " collided!\n";
+                }
+            });
+        });
+
+    // Create a few test entities
+    for (int i = 0; i < 10; i ++) {
+        ecs.entity()
+            .set<Position>({ randd(100), randd(100) })
+            .set<Radius>({ randd(10) + 1 });
+    }
+
+    // Run the system
+    sys.run();
+
+    // Output
+    //  526 and 529 collided!
+    //  527 and 534 collided!
+    //  529 and 531 collided!
+    //  533 and 535 collided!
+}