The ability to set a Intel "Base Platform Toolset" older than the latest version seems to have got broken/lost with the Visual Studio 2017 integration in ICC 2017 Update 4.It only shows v141 and v141_xp (v141 is the VS 2017 toolset)
This seems to be a Visual Studio 2017 integration issue, if I open the same project in VS 2015, the Intel "Base Platform Toolset" shows a full list of possible options back to the oldest version of VS installed on my computer.
This is a very annoying problem as one of the great features of Visual Studio (2017) is the ability to non-destructively open older projects and build them using the older toolsets. This is now broken if the project is an Intel Compiler project.
Please see here (https://blogs.msdn.microsoft.com/vcblog/2017/11/15/side-by-side-minor-version-msvc-toolsets-in-visua... . I believe this will allow you to change the base toolset, though the work required is a bit more than just flipping a switch.