Win32: S_ISLNK doesn't exist (difference in symlink implementation)

Symbolic links on Windows are different to that on POSIX systems. S_ISLNK does
not exist, but should not be needed on Windows anyway.
Use gnulib for strtok_r. See PR #1475 for more info.

This restores Windows XP compatibility.
Merge pull request #1639 from jtanx/win32

Win32: Compilation fixes