Adopting Windows VS project files to work with VS2012/2013

VS 2013 complains that every single file should be "included" only once, which means on human language that every files should have a single build rule. If we want to build two objects out of a single source, we should do it as a single action. This change-set does exactly this. Now ISPC can be build by VS2010SP1/VS2012/VS2013.