Properly link flecs library

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

@@ -0,0 +1,63 @@
+#include <custom_runner.h>
+#include <iostream>
+
+// Systems can be created with a custom run function that takes control over the
+// entire iteration. By default, a system is invoked once per matched table, 
+// which means the function can be called multiple times per frame. In some
+// cases that's inconvenient, like when a system has things it needs to do only
+// once per frame. For these use cases, the run callback can be used which is
+// called once per frame per system.
+
+struct Position { 
+    double x, y;
+};
+
+struct Velocity { 
+    double x, y;
+};
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    flecs::system s = ecs.system<Position, const Velocity>()
+        // The run function has a signature that accepts a C iterator. By 
+        // forwarding the iterator to it->callback, the each function of the
+        // system is invoked.
+        .run([](flecs::iter_t *it) {
+            std::cout << "Move begin\n";
+
+            // Walk over the iterator, forward to the system callback
+            while (ecs_iter_next(it)) {
+                it->callback(it);
+            }
+
+            std::cout << "Move end\n";
+        })
+        .each([](flecs::entity e, Position& p, const Velocity& v) {
+            p.x += v.x;
+            p.y += v.y;
+            std::cout << e.name() << ": {" << p.x << ", " << p.y << "}\n";
+        });
+
+    // Create a few test entities for a Position, Velocity query
+    ecs.entity("e1")
+        .set<Position>({10, 20})
+        .set<Velocity>({1, 2});
+
+    ecs.entity("e2")
+        .set<Position>({10, 20})
+        .set<Velocity>({3, 4});
+
+    // This entity will not match as it does not have Position, Velocity
+    ecs.entity("e3")
+        .set<Position>({10, 20});
+
+    // Run the system
+    s.run();
+
+    // Output
+    //  Move begin
+    //  e1: {11, 22}
+    //  e2: {13, 24}
+    //  Move end
+}