Add GekkoNet as networking library

Hey!

Ive been working on GekkoNet for 2 years now and its pretty much feature complete now. I thought it might be good to open an issue to see if GekkoNet also could be included. GekkoNet is an open source c/c++ rollback networking sdk akin to GGPO but with alot of fixes which makes it generally easier to work with and makes it crossplatform out of the box.

its been used various projects such as 3sx - Street Fighter 3rd Strike PC port https://github.com/crowded-street/3sx

and various emulator projects such as bsnes netplay: https://github.com/HeatXD/bsnes-netplay

and n64 netplay: https://github.com/Jay-Day/RMG-K

Have a nice day! Heat

https://github.com/HeatXD/GekkoNet