Hi,

I am writing code for a new distribution and would like to include some functions from blas (I have already got implementations of the distribution and gradients written in a very efficient and robust manner using blas).

At present I am able to compile the new_normal distribution example Stan Math Library: Probability Distributions in a way that I am happy with. As a very minimal working example I have replaced the code to calculate the log density with a very simple function in the file calc_log_like.hpp

```
double calc_log_likelihood(const int n_y, const int n_mu, const int n_sigma, const double* y,const double* mu,const double* sigma){
double l_sum = 0.0;
double y_copy [n_y];
for (int i = 0; i < n_y; ++i){
l_sum += -0.5 * std::pow((y[i] - mu[i]) / sigma[0], 2);
}
return l_sum;
}
```

While this code isn’t very general (it only works when mu and y are vectors of same length and sigma is a scalar it does let me test what I want. I can do

```
#include "calc_log_like.hpp"
```

inside new_normal.hpp and call the function to calculate the log density. This compiles and works fine during sampling.

Next, the simplest test of using a blas function I could think of was to use the dcopy() so I modified my function to be

```
extern "C"
{
#include <cblas.h>
}
double calc_log_likelihood(const int n_y, const int n_mu, const int n_sigma, const double* y,const double* mu,const double* sigma){
double l_sum = 0.0;
double y_copy [n_y];
cblas_dcopy(n_y, y, 1, y_copy, 1);
for (int i = 0; i < n_y; ++i){
l_sum += -0.5 * std::pow((y_copy[i] - mu[i]) / sigma[0], 2);
}
return l_sum;
}
```

I then use the make/local file with the following line

```
CXXFLAGS+= -L /usr/lib/x86_64-linux-gnu/atlas/ -lcblas
```

I am using ubuntu and have used apt-get install libatlas-base-dev to get blas

When I try to compile the model file I get the error

```
in function `calc_log_likelihood(int, int, int, double const*, double const*, double const*)':
ssm.hpp:(.text+0x430): undefined reference to `cblas_dcopy'
```

How can I link the blas functions correctly?