sci-mathematics/eclib: new package for rational elliptic curves.

sci-mathematics/eclib: new package for rational elliptic curves.

This commit imports eclib-20190909 from the sage-on-gentoo overlay
where François Bissey has been maintaining it for use with SageMath.
I've made a few changes on top of the overlay version:

  * Enabled the test suite (it passes for me).
  * Put the "--disable-allprogs" configure flag behind USE=minimal.
  * Put the "--with-boost" configure flag behind USE=boost.
  * Inlined the SRC_URI which was using a custom thirdpartymirror.
  * Dropped obsolete autotools.eclass and RESTRICT=mirror.

The SageMath build system can detect eclib on the system now, so having
this package installed will save users time when rebuilding SageMath.

Closes: https://bugs.gentoo.org/704096
Package-Manager: Portage-2.3.79, Repoman-2.3.16
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>