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Microsoft releases MS-DOS 4 source code on GitHub — 45 year old code now open-source
(www.tomshardware.com)
Discussions on vintage and retrocomputing
This explains it, although it is not really Git's fault as that article suggests, but rather the charset conversions to UTF-8 that broke things. With all that fixed it builds fine. I've been using DOSBOX and since all the required build tools are included in the repo, it is easy to build.