Well. I now have a hatari_libretro.so 2.21 core running in RetroArch on Ubuntu x64. It's not getting ticks as it should just yet, but I'm confident I can get this to work.
The original fork has quite a few patches. Some of them are in "core" Hatari files (mostly main and statusbar) and can be ifdef:ed in a not-too-beautiful way. The rest are mostly changes to the SDL UI to better conform to how the RetroArch framework needs things to be.
The v1.8 version used its own Makefile to compile everything. I've taken the route of using Hatari's cmake to compile, and then just finish up the last few bits and link using a separate Makefile for libretro.
When all is working I'll send a diff. It would be nice to upstream as much as possible, and I think some compile time config options to the SDL UI will go a long where there.
regards,
Troed