Fix compatibility with Gentoo's virtual/cblas

Previous versions of this patch:
  - https://dev.gentoo.org/~sam/distfiles/sci-libs/gsl/gsl-2.7-cblas.patch.bz2
  - https://dev.gentoo.org/~mgorny/dist/gsl-2.3-cblas.patch.bz2
  - https://dev.gentoo.org/~mgorny/dist/gsl-1.16-cblas.patch.bz2

Co-authored-by: Sam James <sam@gentoo.org>
Co-authored-by: Michał Górny <mgorny@gentoo.org>
Signed-off-by: Aliaksei Urbanski <aliaksei.urbanski@gmail.com>

preparing 2.8 release