it‘s quite easy. The darktable build process is well documented. So you just need a little bit of git skills to merge the mentioned pull request into your libraw submodule folder.
Or use my R6mIII related branch (GitHub - MStraeten/LibRaw at r6mIII · GitHub) to copy the different files from there…
If you’re on macOS you might use my recent builds (current OSX Build - #973 by MStraeten)