Using Paho MQTT in OMNeT++ Simulation

I am integrating the Paho MQTT C++ library into an OMNeT++ implementation. I copied over the source directories and use the custom make files of the projects for building the C and the C++ libraries inside OMNeT++. I am using some test code which uses the code shown here:

#include "MQTTConnector.h"

Define_Module(MQTTConnector);

const string SERVER_ADDRESS { "tcp://localhost:1883" };
const string CLIENT_ID      { "sync_client" };
const string TOPIC          { "hello" };

const int  QOS = 1;

class callback : public virtual mqtt::callback
{
    mqtt::client& cli_;

    void connected(const std::string& cause) override {
    std::cout << "\nConnected: " << cause << std::endl;
    cli_.subscribe(TOPIC, QOS);
    std::cout << std::endl;
}

void connection_lost(const std::string& cause) override {
    std::cout << "\nConnection lost";
    if (!cause.empty())
        std::cout << ": " << cause << std::endl;
    std::cout << std::endl;
}

void message_arrived(mqtt::const_message_ptr msg) override {
    std::cout << msg->get_topic() << ": " << msg->get_payload_str() << std::endl;
}

void delivery_complete(mqtt::delivery_token_ptr token) override {}

public:
    callback(mqtt::client& cli) : cli_(cli) {}
};

void MQTTConnector::initialize() {

    mqtt::connect_options connOpts;
    connOpts.set_keep_alive_interval(20);
    connOpts.set_clean_session(false);
    connOpts.set_automatic_reconnect(true);

    client = new mqtt::client(SERVER_ADDRESS, CLIENT_ID);

    callback cb(*client);
    this->client->set_callback(cb);

    try {
        cout << "Connecting to the MQTT server..." << flush;
        this->client->connect(connOpts);
        cout << "OK" << endl;
    }
    catch (const mqtt::exception& exc) {
        cerr << exc.what() << endl;
    }
}

First, I tried the asynchronous client, which worked fine. However, I rather want to use the synchronous client. With that I get the following error, despite the fact, that I link the needed libraries with the options -lpaho-mqtt3c and -lpaho-mqttpp3

 Undefined symbols for architecture x86_64:
  "mqtt::client::client(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, mqtt::iclient_persistence*)", referenced from:
      MQTTConnector::initialize() in MQTTConnector.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Any idea what is wrong here?

EDIT: I just tried to compile and install the libraries in the regular way on my machine (macOS 10.14). Unfortunately, this does not change the behavior.

I fixed it by setting an absolute path as linker flag. Before I set -L relatively in the .oppfeatures file for my MQTT feature. The solution was to set it to pwd and traverse to the correct library folder from this directory.