dosbox-unstable: init at 2017-07-02

As current stable version segfaults when playing HoMM2, as described at
https://www.reddit.com/r/linux_gaming/comments/4dxfei/dosbox_segmentation_fault_core_dumped/

Also some missing dependencies (compared to stable version) were added:
- SDL_sound - for mounting .cue files with compressed sound
- SDL_net - for IPX support
- libpng - for making screenshots