modifying patch files

[Jack Howarth]

   I have run into an issue that is unclear on MacPorts. In fink
we often would put placekeepers in a patch such as @FINKPREFIX@
and use the command...

sed 's|@FINKPREFIX@|%p|g' <%{PatchFile} | patch -p1

to replace the @FINKPREFIX@ with the actual fink installation directory.
How do you handle this situation in MacPorts where you want to
generalize the MacPorts prefix in a Makefile patch for instance
to survive the case of a user installing MacPorts in a non-default
location other than /opt/local?
                   Jack