ENH: Respect blas_opt and lapack_opt groups in site.cfg.

_Do not merge (yet)._

Previously blas_opt and lapack_opt would only be merged into the autodetected BLAS and LAPACK libraries. This prevented using custom LAPACK and BLAS implementation if a system installed version of ATLAS (or similar) was present.

Now the presence of a blas_opt or lapack_opt section completely overrides the respective autodetection mechanisms, as the `site.cfg` documentation would already lead you to believe.

I'm opening this for discussion.  There's a lot more backstory and some basic scripts (to be used to test the behavior of the autodetection) in  #2728.  I'm not convinced in any way that this is the "correct" approach to the problem.  Also it doesn't appear that the `language` parameter can be set this way -- but is it even necessary to set?