Visual Studio 2010 Projects: Add PlatformToolset Tag

This adds a PlatformToolset tag for each configuration so that it is easier
to upgrade the projects to Visual Studio 2012/2013 formats, which are
largely the same as the Visual Studio 2010 projects.  We can, for example,
use a script to change the values of the PlatformToolset to make these
projects usable out-of-the-box for Visual Studio 2012/2013.