how to prepend or append to DYLD_LIBRARY_PATH in Portfile?

[Ken Cunningham]

It's easy to prepend or append to PATH in a Portfile, but I'm having difficulty seeing how to modify, rather than overwrite, DYLD_LIBRARY_PATH.

Usually you just want to set it, but now that it might or might not be set in legacysupport 1.1, I was hoping to just modify rather than overwrite it.

I tried a number of different things, but couldn't spot the magic incantation...am 
i missing something simple?

Thanks, Ken