since I am forced to build Blender myself every time there’s a new version or hotfix released because the select through patch keeps getting ignored, I keep running into all sorts of build system issues I am unable to resolve myself.
Sorry can’t help you there. The patch isn’t in an acceptable state. I wouldn’t say that devs are ignoring the patch, but have other priorities. If you want a patch to move forward try to see if you can start on discussing the design.
Now back to your original question. It tries to apply a patch (T70357) on top of latest, but the it cannot be applied anymore as there is a conflict. You first have to fix the conflicts and then you can continue.
See here for a short tutorial about resolving conflicts. 30. Resolving Conflicts
You can open the mentioned file and try to detect the changes and what needs to be done to resolve them.
The issue here is that I don’t have necessary experience with the technicalities of the build system, but I am forced to deal with it despite that because I am relying on that patch. I have no choice here.
Anyway, my point is that if I deleted absolutely everything, the entire folder with Blender repo, generated project and libraries, and started completely from scratch, following this guide step by step Building Blender/Windows - Blender Developer Wiki I should not get to a point with that merge error, should I?
I mean the guide should allow anyone to build Blender, yet it doesn’t contain any info about the merge conflict. I mean checking out the 2.93.4 version by the tag should just work, shouldn’t it?
Solving merge conflicts is a development task, not a building task hence it isn’t documented there. The solution involves updating the patch you want to apply. Normally a developer should working in the area should be able to solve these conflicts. I would suggest to check with an add-on developer how to solve the merge conflict. Or reach out to the person who added the patch to update it to reflect recent code changes
I’m guessing your Blender git repository is in a detached HEAD state (meaning, not in any branch), and that make update is unable to identify which release it should use. git status will tell you if that’s the case.
We can modify make update to give a more helpful error message, but it won’t fix your problem. I think it would be best to checkout the blender-v2.93-release branch and commit your patch locally in that branch.