undefined reference to `omp_get_num_procs'

# include <cstdlib>
# include <iostream>
# include <iomanip>
# include <omp.h>

using namespace std;

int main ( int argc, char *argv[] );

//********************************
int main ( int argc, char *argv[] )
//**********************************
{
  int id;
  double wtime;

  cout << "\n";
  cout << "HELLO_OPENMP\n";
  cout << "  C++/OpenMP version\n";

  cout << "\n";
  cout << "  Number of processors available = " << omp_get_num_procs ( ) << "\n";
  cout << "  Number of threads =              " << omp_get_max_threads ( ) << "\n";

  wtime = omp_get_wtime ( );

# pragma omp parallel \
  private ( id )
  {
    id = omp_get_thread_num ( );
    cout << "  This is process " << id << "\n";
  }
  wtime = omp_get_wtime ( ) - wtime;

  cout << "\n";
  cout << "HELLO_OPENMP\n";
  cout << "  Normal end of execution.\n";

  cout << "\n";
  cout << "  Elapsed wall clock time = " << wtime << "\n";

  return 0;
}

Hello, I try to run this program:

I run make "filename" and I had this error:

g++ hello_openmp.cpp -o hello_openmp /tmp/ccz80Tfg.o: In function main': hello_openmp.cpp:(.text+0x4d): undefined reference toomp_get_num_procs' hello_openmp.cpp:(.text+0x7a): undefined reference to omp_get_max_threads' hello_openmp.cpp:(.text+0xa7): undefined reference toomp_get_wtime' hello_openmp.cpp:(.text+0xb9): undefined reference to omp_get_thread_num' hello_openmp.cpp:(.text+0xea): undefined reference toomp_get_wtime' collect2: error: ld returned 1 exit status make: *** [hello_openmp] Error 1

Is anyone knows what is the meaning of this error. Sorry I am beginner in C++. Thank you