Path conversion if runing on cygwin

This patch will detect if emacs is runing on cygwin windows and will translate the ```buffer-file-name``` from POSIX style to windows style.

eg: ```/cygdrive/c/test.dot -> C:\test.dot```

I've started studying LISP yesterday and I'm not so skilled, so this is half a pull request and half an help request: the compile and preview commands works as expected, but the ```graphviz-dot-view-command``` do not work properly.

Given a ```buffer-file-name``` like ```/cygdrive/c/folder/test.dot``` it will invoke ```doted C:folder:test.dot``` instead of ```doted C:\folder\test.dot```.

I've tested it on linux and it still works.