It means you're on Cygwin, or perhaps midipix, or maybe even winelib.
EDIT: I'll note that I'm about as fond of these umbrella feature flags as I am of user-agent sniffing. Midipix and Cygwin support Unix paths, but have to deal with Windows' idea of reserved characters; meanwhile Winelib allows anything the underlying FS supports. On the other hand, Midipix and Winelib can use Windows GUI constructs, while Cygwin cannot.