sci-mathematics/gfan: new version 0.6.2.

sci-mathematics/gfan: new version 0.6.2.

Update our version of gfan to the latest upstream v0.6.2, from
September 28th 2017. The new ebuild is based off of the version that
François Bissey maintains in the sage-on-gentoo overlay, with a few
additional changes:

  * Merged the EAPI=7 and slot dependency updates from ::gentoo.

  * Change the LICENSE to "GPL-2+", to match the upstream LICENSE file.

  * Stripped some more CFLAGS mangling out of the upstream Makefile.
    Corresponding to this we now replace "-O3" by "-O2" in the ebuild.

  * Added USE="doc", to build and install the LaTeX manual.

Closes: https://bugs.gentoo.org/711492
Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>