Beginner Question: Best way to substitute variables

[Frank Behrens]

Meanwhile I use FreeBSD for several years and now comes the time to 
give something back. I'm creating a port and could not find an answer 
to my question:

What is the best way to set variables in scripts (e.g. pkg-install) 
and text files (e.g. pkg-message) depending from Makefile variables 
(e.g. PREFIX)? 
I understand the Makefile rules and it is no problem to use variable 
paths depending from $PREFIX, $DOCSDIR and other, but how they are 
later used in scripts, like pkg-install and rc.d/scripts? IMHO 
changing these and other variables in the Makefile should be enough 
to adjust the complete port.

As a possible solution I see:
Put all files in a separate directory, create Makefile dependencies 
and replace the desired strings with "sed". But this seems to be not 
the usual way, I could not find any example port.

Please, could you point me to an example port, where this problem has 
been solved in a practical way conform to FreeBSd development rules.

Best Regards,
   Frank