Provides two options when building RPCS3
USE_SYSTEM_FFMPEG BOOL (default: OFF)
USE_SYSTEM_LIBPNG BOOL (default: OFF)
These options lets the user select between the system provided and builtin libraries of ffmpeg and libpng to overcome possible system issues.
Also adds support for older libpng releases if the user doesn't have libpng 1.5 or higher.
* Error codes are now properly handled
* Couple more functions implemented
* Support for setting certain socket information
* socketselect() handles Unix waiting behaviour on Windows
* accept() fixed