Link errors when building/testing

[canndrew]

I'm on Gentoo and have the sci-libs/cblas-reference and virtual/cblas packages installed. When I run cargo test I get a couple of pages of errors "undefined reference to `cblas_...'"

I eventually figured out that I need to build with CARGO_BLAS=cblas to make it work.

[robsmith11]

I needed this workaround as well on Arch Linux.