Recently VTK-m was changed to require C++11. The internal builds set
properties to require C++11, but these never make it to the
configuration for projects that use the VTK-m package (i.e. not declared
in VTKmConfig.cmake).

This change adds a new CMake target, vtkm, which is an interface. It
does not point to an actual library, but it allows code that links it in
to have the appropriate compile flags.