(This probably says more about how hard it is to build C++ than anything else)
May not take hours, but determined engineer should be able to figure it out.
Just getting the code to compile, link, baking in assets, etc. For a single architecture is a much more reasonable goal.
I'd imagine it'd be on the scale of 1-3months for an engineer to get working full time, but large error bars around this figure
This was done (if I remember right) when governments and big customers had access to the Windows source-code.
That's probably true, but I wouldn't be surprised if something like windows doesn't have a README file. And it does have build instructions they may well be in some wiki separated from the source code.
Video games build systems are a thing onto themselves even taking the c++ issues out of the equation.
Hell even comparatively simole microservices in modern CRUD apps have resorted to docker to do away with pains of rebuilding stuff.