gens-gs 2.16.7, patch level 2 (linked against SDL2-i386)

I've tested this new version with 

```
antimicro --hidden --profile /home/yo/RetroRig/gamepad-cfgs/ps3_blu_controller/am_gensgs_ps3_blu.joystick.xml 2>/dev/null &
export SDL_VIDEO_FULLSCREEN_HEAD=1
export HOME=/home/yo/.retrorig
gens --fs /home/yo/RetroRig/ROMs/Sega\ Genesis/Sonic\ the\ Hedgehog.bin 2>/dev/null 1>/dev/nul
```
and with both `Normal` and `Super Eagle`in gens.cfg. It ran flawless on my rig. Load state / save state and game exit work. GensGS doesn't need the --quick-exit option anymore, it's permanently switched on, and react on ESC, just like in all other emulators.

From a feature standpoint, it only adds dual monitor support to the former gens-gs version. That has been tested as well of course.

I little bit to the background of this binary:

It's based on gens-gs-r7.tar.gz. The last changelog entry is

```
gens (2.16.7) unstable; urgency=low

  * Release 7 version bump.

 -- David Korth <gerbilsoft _at_ verizon.net>  Sun, 22 Nov 2009 13:17:35 -0500
```

It can be found originally on [gerbilsoft](http://www.soniccenter.org/gerbilsoft/gens/r7/gens-gs-r7.tar.gz), and now also on [libregeek.org](http://www.libregeek.org/RetroRig/Ubuntu-Trusty/emulators/gens/) and in my [github repository](https://github.com/beaumanvienna/gens-gs).

The github project currently comprises three branches:

* *original_gens-gs-r7*, which is the original software
* *retrorig-pl1*, which is almost the original software, except for a deleted drag and drop function. It compiles on recent Debian derivatives and is linked against SDL 1.2
* and *retrorig-pl2*, which I now ported to SDL 2.

There is a more recent version of gens-gs located at [github/lutris](https://github.com/lutris/gens/commits/master), it seems that gens development was continued for some month after the release of R7 (2.16.7), but that this work was not released. Or maybe it was, there is a [version 2.16.7-4 available for Arch](https://www.archlinux.org/packages/multilib/x86_64/gens-gs/).


When it comes to RetroRig and the deployment of gens, I like to suggest that we associate `*.smd` files with it. I also prefer it over mednafen for `*.bin`-files, mainly for the "Super Eagle" rendering interpolation, but also for less display tearing.

This pull request basically covers issue #117 "GensGS support".