Missing `snprintf` on Windows

Last time `snprintf` was introduced while making a workaround for Windows-only functions like `_itoa_s` and `CString`. I was surprised to found this function was [still not included](http://stackoverflow.com/questions/2915672/snprintf-and-visual-studio-2010) in Visual Studio.

Here is a patch for (hopefully) making it run again on Windows. Soo, thank you for reporting this issue.