BLAS linking

Rather than linking against -lopenblas, we should be linking against -llapack -lblas. This should allow us to change the BLAS implementation at runtime, as outlined in the conda-forge docs.

https://conda-forge.org/docs/maintainer/knowledge_base.html#blas

Note that this also includes use of the macOS Accelerate framework, via the conda-forge libblas=*=*accelerate" packages.