Undefined reference compiling program with library "jrtplib"

Question

Compiling the following program

#include "jrtplib3/rtpsession.h"
#include "jrtplib3/rtpudpv4transmitter.h"
#include "jrtplib3/rtpipv4address.h"
#include "jrtplib3/rtpsessionparams.h"
#include "jrtplib3/rtperrors.h"
#include "jrtplib3/rtplibraryversion.h"
#include <stdlib.h>
#include <stdio.h>
#include <iostream>
#include <string>

using namespace jrtplib;

void checkerror(int rtperr)
{
    if (rtperr < 0)
    {
        std::cout << "ERROR: " << RTPGetErrorString(rtperr) << std::endl;
            exit(-1);
    }
}

int main(void){
    return 0;
}

with

g++ -o ex1 ex1.cpp

throws me the following error:

/tmp/ccDU6m7Y.o: In function `checkerror(int)':
ex1.cpp:(.text+0x2e): undefined reference to `jrtplib::RTPGetErrorString[abi:cxx11](int)'
collect2: error: ld returned 1 exit status

even though my LD_LIBRARY_PATH is set to /usr/local/lib and the folder contains the necessary libraries:

libjrtp.a
libjrtp.so
libjrtp.so.3.11.1

I compiled jthread as well as jrtplib with

mkdir build
cd build 
cmake ..
sudo all make

I also figured out that the examples from the jrtplib compiled successfully. My code example is a snippet from example1. Unfortunately, I couldn't figure out where the difference lies. I think the answer to my problem is quite simple.