force setting cmake predefind VTK dir

[fmigneault]

I encountered error https://github.com/PointCloudLibrary/pcl/issues/4926 when trying to link PCL with another library when it was compiled using a self-provided VTK (same as in issue).

I found 2 workarounds. First, being to NOT provide VTK_DIR to cmake (or remove from cache if already there even if empty) and leaving PCL set it. The second is to FORCE setting the value, basically ignoring what was provided by the user.

Since the displayed error happens when VTK_DIR is specified, it is safe to assume it is already too late for case 1.

fixes #4926