I've wanted to do this too.  Emacs needs a patch to make it play well
with cfs and coda.  Well, probably cfs and coda need fixing really,
but I'd like to have a PKG_OPTIONS.emacs=coda be able to pull in a
patch that disables clash detection, since those symlinks end up
causing trouble.  Off by default, of course.   There's a similar fix
for cfs, but it's always applied, since it doesn't hurt.

I think one can conditionally add a PATCHFILE, perhaps from files/.

I see the point about preferring to add patches with defines. 

-- 
        Greg Troxel <gdt@ir.bbn.com>