Umm.. not compression artifacts. GIF uses lossless LZW. Maybe color palette artifacts since GIFs are usually palettized and not true color (although with a tortured use of local color tables they can even be true color)
Libjpeg does have support for it. Unfortunately, you have to have two copies of the library, one for 8 bit and one for 12. And you have to rename all the API methods in one of the libraries so you dont get name collisions. I believe that LibTIFF has a build configuration for this so that 12bit JPEG data can be encapsulated in a TIFF file.