Allow flexiblas #8

Not sure if it would require something special, I am thinking an extra line to search for those libs, but having flexiblas as an option could be useful. It's a drop-in replacement and underneath dispatches everything to the correct implementation.

It is namely used by the canadian computer cluster on their AMD processor stack (where the intel mkl is likely subpar/not working), more info here samuelstjean/spams-python#20