While I was creating the patch, I had the same feeling than @dkazakov: I'm not sure it's better with so many lines duplicated. It's messier to maintain.

Ok, thanks! I'll change the patch accordingly.

I also thought about splitting the build instructions into three files, but I'm not sure if it's a good idea. Anyway, since I'm new to Krita's development, I am aware I can suggest wrong ideas, so feel free to ask for improvement :)

I've created a copy of Krita's Git repo on Bitbucket. You can preview the 3rdparty/README.md file at https://bitbucket.org/ablondin/krita-private/src/8f0fc07084f16e89f47d4f0716e79d242adfaea8/3rdparty?at=refactoring-3rdparty-readme. If you want me to remove outdated parts in my patch, I can do it without problem, but I haven't identified any for now. In particular, I don't have access to any Windows machine, so that I can't test it (just MacOS for now, but I also have Debian at work, which I didn't test so far).