[Paraview] Add support for PARAVIEW_EXTRA_EXTERNAL_PLUGINS?

[John Biddiscombe]

> PARAVIEW_EXTRA_EXTERNAL_MODULES is useful when deploying ParaView with
>  extensions on a platform that does not support shared libraries (for
> example Cray Xts and IBM Blue Genes).
>   

And also to clarify. I did not intend to remove the 
PARAVIEW_EXTRA_EXTERNAL_MODULES, only to add PARAVIEW_EXTRA_EXTERNAL_PLUGINS

JB