Added Lapack in linalg.h and a fixed a minor #elif.

Also, the signature is
```c++
double cblas_ddot(const int N, const double *X, const int incX, const double *Y, const int incY);
```
So passing `shogun::SGVector<T>` directly will cause disaster ! We gotta make use of `SGVector` 's `vec` and `vlen` here. Please test the code locally before pushing :-)