Support Libretro API for cross-platform porting

Libretro is an API for exposing core emulator functions (video/audio/input/serialization) so that an emulator may be easily ported through cross-platform frontends such as RetroArch.

The following emulators already have mainline support for libretro:
BSNES (unofficial git) - http://gitorious.org/bsnes/bsnes/commits/libretro
GenPlus-GX - https://code.google.com/p/genplus-gx/source/browse/trunk
FB Alpha - http://www.barryharris.me.uk/fba.php
Meteor GBA - http://sourceforge.net/p/meteorgba/code/ci/96191a9262a1752660d8e2e5816add9674a917cb/tree/
FCEUmm - http://code.google.com/p/fceumm/source/browse/#svn%2Fsrc%2Fdrivers%2Flibretro

Plus a huge amount of supported ports: https://github.com/libretro

More details on RetroArch/libretro here:

http://www.libretro.org/
https://github.com/Themaister/RetroArch