Minor comments:

Instead of or as well as Win32::GetOSVersion I'd say provide the result of `ver`.

Neither LIB nor INCLUDE are gcc specific on Win32. They are standard for VC as well.

Instead of umask, it might be interesting on Win32 to see the results of stuff like cacls.

Knowing cwd() would be useful. As would knowing the full paths to things like (n)make, and other tools. Sometimes failure could be occuring because the tools have spaces in their names.

On win32 knowing if the user is an administrator/power user or not would be useful.

Locale settings might be nice. For instance code might be failing because the locale has strange settings for the thousands seperator or the decimal point.

If it was possible knowing the machine localtime as well as a reference time would be good. Although i could imagine this would be a difficult one.

As for prioritization, I'd say that the more info provided the better so prioritize the easiest to add, and defer the harder until the rest are done.