sci-libs/linbox: new C++ template library for linear algebra.

sci-libs/linbox: new C++ template library for linear algebra.

This is a straightforward import of the sci-libs/linbox package from
the sage-on-gentoo overlay where François Bissey has been maintaining
it. Linbox is a dependency of SageMath, and users will soon be able to
use the system copy of linbox; see ticket 29631 on the SageMath Trac
instance. Having this in Gentoo will therefore avoid pointless
rebuilds of linbox for our SageMath users.

Closes: https://bugs.gentoo.org/227813
Package-Manager: Portage-2.3.89, Repoman-2.3.20
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>