Properly link flecs library

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

@@ -0,0 +1,11 @@
+#include <app.h>
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    ecs.system().iter([](flecs::iter& it) {
+        it.world().quit();
+    });
+    
+    return ecs.app().run();
+}