Support Building Using Visual Studio 2013

Like the Visual Studio 2012 project files, the Visual Studio 2013 files are
largely the same as the Visual Studio 2010 project files, so support
Visual Studio 2013 by updating the autotools scripts that is used for
Visual Studio 2012.  This means that project files for Visual Studio 2012
and Visual Studio 2013 can be maintained by simply maintaining the Visual
Studio 2010 project files, adding minimal maintenance overhead.