I'm curious what the preferred way to handle this situation as well.
I notice that several packages, e.g. editors/xemacs20, use make
variables from /etc/mk.conf to conditionally compile certain features
into the package.  I am putting together a new Window Maker package
which does something similar to conditionally compile support for KDE
or GNOME, plus various I18N files.  Is this the right way to do it?

I imagine the situation only gets difficult when the various compile
options cause different files to be installed, meaning we have to
cobble together the correct PLIST from several files.
--
   Johnny C. Lam
   Department of Statistics                 lamj@stat.cmu.edu
   Carnegie Mellon University               http://www.stat.cmu.edu/~lamj/

                                @>---`---,----