sci-mathematics/gp2c: new package to compile GP routines to C.

sci-mathematics/gp2c: new package to compile GP routines to C.

This commit imports version 0.0.11.2 more or less as-is, from the
sage-on-gentoo overlay where François Bissey has been maintaining it
for use with SageMath. I've enabled the test suite, which passes for
me. I also moved dev-lang/perl into BDEPEND, since it's run on the
build host to compile function descriptions.

Closes: https://bugs.gentoo.org/703488
Package-Manager: Portage-2.3.79, Repoman-2.3.16
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>