How to make installation conditional in pkg-plist

[Montgomery-Smith, Stephen]

Currently the ports math/vtk6 and science/paraview both try to install 
the same file: share/cmake/hdf5/libhdf5.settings.  It is described in 
this bug report.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=211334

My proposed solution is that each port only installs the file if it 
doesn't already exist, and only deinstalls the file if it installed it. 
I am planning to figure out some kind of @exec solution for the pkg-plist.

Has anyone already solved this problem in other situations?  I prefer 
not to reinvent something if good solutions already exist.

Failing that, does anyone have good ideas on how to do this, or even 
alternative solutions?

Thanks, Stephen