NEW: math/gap

From pkg/DESCR-main:

GAP is a system for computational discrete algebra, with particular
emphasis on Computational Group Theory. GAP provides a programming
language, a library of thousands of functions implementing algebraic
algorithms written in the GAP language as well as large data libraries
of algebraic objects.

I split up the package into -main, -docs, and -pkgs. This is kind of
copying what texlive does, since there is a "core" that GAP needs to
function and a large assortment of other packages that most people
won't want.

The -pkgs package is 1G when installed while the entire rest of GAP
(the whole programming language, core libraries, some databases of
objects) is only 0.5G.

I build GAP without readline since I wasn't able to wrestle autotools
into detecting the readline from ports correctly. There is a pull
request to fix this (https://github.com/gap-system/gap/pull/3428), but
I am no autotools expert (or even intermediate) and I couldn't get it
to work, so that patch is not included.

Comments and improvements appreciated.

--
Kaashif Hymabaccus
GPG: 3E810B04