newbie dnn module problem with c++

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Hello,

I compiled and installed opencv 3.4.1. When i write a small c++ program which reads in a torch model, it fails compiling with

CMakeFiles/nettest.dir/nettest.cpp.o: In function `main':
nettest.cpp:(.text+0x46): undefined reference to `cv::dnn::experimental_dnn_v4::readNetFromTorch(cv::String const&, bool)'
nettest.cpp:(.text+0x8b): undefined reference to `cv::dnn::experimental_dnn_v4::Net::~Net()'
nettest.cpp:(.text+0xc7): undefined reference to `cv::dnn::experimental_dnn_v4::Net::~Net()'
CMakeFiles/nettest.dir/nettest.cpp.o: In function `cv::String::String(char const*)':
nettest.cpp:(.text._ZN2cv6StringC2EPKc[_ZN2cv6StringC5EPKc]+0x54): undefined reference to `cv::String::allocate(unsigned long)'
CMakeFiles/nettest.dir/nettest.cpp.o: In function `cv::String::~String()':
nettest.cpp:(.text._ZN2cv6StringD2Ev[_ZN2cv6StringD5Ev]+0x14): undefined reference to `cv::String::deallocate()'
collect2: error: ld returned 1 exit status
CMakeFiles/nettest.dir/build.make:94: recipe for target 'nettest' failed

The code:

#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/dnn.hpp"
#include <math.h>
#include <iostream>
using namespace cv;
using namespace std;
using namespace dnn;

int main( int argc, char** argv)
{
  Net net = readNetFromTorch("openface.nn4.small2.v1.t7");  
  cout << "main" << endl;
  return 0;
}

Did i made something wrong during compiling / installing opencv or is there a stupid mistake in my program? Thank you very much + Greetings, Holger