The BLAS implementations (reference, openblas, atlas) all provide
an alternative for blas.pc, slaved to the alternative for
libblas.so-<arch>.
But the libblis*-dev packages do not provide this alternative for
blas.pc.
This means that libblis-dev cannot be considered as a drop-in
alternative BLAS implementation.
Client packages that use pkg-config to configure their BLAS support
are broken by BLIS.
Hence the libblis*-dev packages need to provide a blas.pc file and
include it in the alternatives configuration.