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

@srgnuclear I'm afraid if this is the only addition you've made for lapack backend support for linalg then it won't work. Notice that if cmake doesn't find eigen3 or viennacl, our friend `HAVE_LINALG_LIB` is still undefined (under which this whole thing works). You haven't taken care of that in the cmake. Maybe I'm missing something here? (any previous PR?)