sci-libs/givaro: new C++ library for arithmetic/algebraic computing.

sci-libs/givaro: new C++ library for arithmetic/algebraic computing.

The latest version givaro-4.1.1 was imported from the sage-on-gentoo
overlay with the following changes:

  * The homepage (which was redirecting) was updated.
  * The obsolete "bindist" flag was dropped.
  * The test suite was enabled; it passes without incident for me.
  * Support for the sse/sse2 CPU flags was added.
  * USE=doc will now build the API documentation.

The SageMath project can detect and use the system copy of givaro, so having
this installed will help Gentoo users avoid pointless rebuilds of givaro.

Closes: https://bugs.gentoo.org/704458
Package-Manager: Portage-2.3.79, Repoman-2.3.16
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>