CONFIG: csh not unsetting old paraview/cmake environment (related to #281)

- On the first call, ParaView_DIR is unset and thus the clean-path
  fails with the warning "ParaView_DIR: Undefined variable."

  This looks messy, but is of no _major_ consequence since paraview
  doesn't need to be removed anyhow. The only slight risk is that the
  path to a third-party cmake might not be cleaned.

- Patch as per Bruno's suggestion.