Ignore PATHEXT on older Windows

[plicease]

There is a comment here that PATHEXT is only WinNT and later, but we honor the variable if it happens to be set, even on Windows9x. We probably should not.

File-Which/lib/File/Which.pm

Line 120 in 2cc0581

# Win9X or other: doesn't have PATHEXT, so needs hardcoded.