Fix config issues with headers and libraries

There were a couple of places where the configure scripts did not add either includes to VTKm_INCLUDE_DIRS or libraries to VTKm_LIBRARIES.

The biggest offender was when the examples used find_package to load the VTK-m configuration it needed. find_package cleared out the includes and libraries, but it did not clear out the VTKm__FOUND variables. Normally, these variables would not be set before find_package is called, but in this case the examples were called after some partial configuration. I got around this issue by clearing out all the *_FOUND variables in VTKmConfig.cmake.