Use platform-specific path separator in makeRelativePath function

I have discussed this issue on the AFDKO mailing list:
https://groups.google.com/forum/#!topic/uafdkoml/2Xpq5ZzAUU8

Basically, the `makeRelativePath` function in MakeOTF.py is enforcing UNIX forward slashes `/` instead of picking the required OS-specific path separator. This makes the ttx command fail when compiling TTF/OTF fonts on Windows -- where a backslash `\` is required instead.

Cheers,

Cosimo