Well, that's true but it could still technically "build", I suppose, what you describe are runtime issues, not build issues. I assume there are other reasons that prevent building with modern compilers?
I would assume that the first problem would be to find a modern compiler that could target 16bit DOS in the first place...
Looks like the code has inline x86 assembly in the Borland format, so that would probably need to be converted for a different C compiler. There are likely to be other, not so obvious dependencies on compiler-specific features.
No to mention the direct access to the VGA which modern Operating System forbid.