Currently, the only library created is for the rendering package. If
VTKm_BUILD_RENDERING is off, then no libraries are created. If no
libraries are created, then there is nothing that declares a VTKmTargets
export. If there is nothing that creates a VTKmTargets export, the
export command fails.

Aaarg!!!! I can't even find a way to query whether an export is valid
(in the same way you can query whether a target exists). I added a
global variable that recorded whether vtkm_library added a library
(where things are added to the VTKmTargets export). The export command
is called if any libraries were created, a stub is created and installed
otherwise.