cygwin pathsep fix

#include \<windows.h\>
defines \_WIN32.
So the subsequent
  #if defined \_\_mingw32__ || defined _WIN32 || defined _MSC_VER
fails for cygwin, leading  at every cpan start to

cygwin warning:
  MS-DOS style path detected: /tmp\\.UUID_STATE
  Preferred POSIX equivalent is: /tmp/.UUID_STATE
  CYGWIN environment variable option "nodosfilewarning" turns off this warning.
  Consult the user's guide for more details about POSIX paths:
    http://cygwin.com/cygwin-ug-net/using.html#using-pathnames