Properly link flecs library

engine/libs/flecs/test/custom_builds/cpp/pipeline/src/main.cpp

@@ -0,0 +1,30 @@
+#include <pipeline.h>
+#include <iostream>
+
+struct Position { 
+    int32_t x, y;
+};
+
+struct Velocity { 
+    int32_t x, y;
+};
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    ecs.system<Position, const Velocity>()
+        .each([](Position& p, const Velocity& v) {
+            p.x += v.x;
+            p.y += v.y;
+        });
+
+    auto e = ecs.entity()
+        .set<Position>({10, 20})
+        .set<Velocity>({1, 2});
+
+    ecs.progress();
+
+    const Position *p = e.get<Position>();
+    assert(p->x == 11);
+    assert(p->y == 22);
+}