When portability is considered, may require a slightly different "version" of each source file for different environments (e.g. Unix, Linux, Windows).

These are called variations rather than versions.

In C, variations are handled via the pre-processor, e.g.

#ifdef UNIX
#include <sys/file.h>
#endif
#ifdef WINDOWS
#include <win/files.h>
#endif

Variations are required for different O/S, different architecture, ...