I've now repackaged both of these and hope they work! (They do here, but that's not always the same thing!)
db46 needs to be processed first as postfix depends on it for building and will fail dependencies if the db is not updated first.
However, postfix can now be built against either (lib)db-devel 4.6 or 4.8.
(I spent the best part of two days trying to build the latest versions (2.8.7, 2.8.8 and 2.9.0) but none of them will build, with or without patches. cp complains a file is not a directory. If I patch the Makefile.in concerned to remove the final filename, ranlib complains the directory is not a file!
Doubtless if I went into detail and spent weeks and got lots of help I would eventually get the job done, but it's not a priority for me as I'm quite happy with the current version for my limited needs (testing websites on my local box where all mail goes to my account on the box irrespective of where it's addressed). If someone needs the latest version, I won't be hurt if they persist and succeed where I have failed.)