Log Message:
libretro-genesis-plus-gx: add version 20180607.

Libretro is a simple but powerful development interface that allows for the
easy creation of emulators, games and multimedia applications that can plug
straight into any libretro-compatible frontend. This development interface is
open to others so that they can run these pluggable emulator and game cores
also in their own programs or devices.

Genesis Plus GX is a Sega 8/16-bit game console emulator focused on accuracy
and portability.

This emulator has 100% compatibility with Genesis / Mega Drive, Sega/Mega CD,
Master System, Game Gear & SG-1000 released software (including all unlicensed
software and known pirate dumps), also emulating backwards compatibility modes
when available. It also has Pico emulation.

It may be somewhat slower than PicoDrive on some platforms, but has improved
accuracy. On the other hand, only PicoDrive supports 32X emulation.