Handle win32 separator for cygwin paths This PR handles a issue that cygwin actually supports Win32 path, so we need to handle the Win32 prefix and separaters. r? `@mati865` cc `@jeremyd2019` ~~Not sure if I should handle the prefix like the windows target... Cygwin *does* support win32 paths directly going through the APIs, but I think it's not the recommended way.~~ Here I just use `cygwin_conv_path` because it handles both cygwin and win32 paths correctly and convert them into absolute POSIX paths. UPDATE: Windows path prefix is handled.