[Help-gsl] First compiling problem

[Manu Hack]

#include <iostream>
#include <gsl/gsl_math.h>
int main(){
       double x;
       x=gsl_pow_2(4);
       std::cout << x << std::endl;
}

I ran an example using GSL.  I added the path after -I when I compiled
it.  I kept getting error.  The error message was: undefined reference
to 'gsl_pow_2.

I could use those constants like M_LOG2E though.

Am I missing anything?  Thanks!