[vtkusers] process.h conflict with windows header

[Ben Boeckel]

On Mon, Jan 15, 2018 at 07:40:08 -0700, jackyko1991 wrote:
> boost::asio includes <process.h> in its header file
> 
> when i use vtk 7 or older, this header point to the windows own header file
> 
> however in vtk8, it points to Utilities/KWSys/vtksys/Process.h.in

Ah, case insensitivity. Try installing VTK rather than using the build
tree. The headers should be put under a `vtksys` directory rather than
being available right in the VTK header directory.

--Ben