i just solved it myself (i'm *sooo* proud...)
reading pgdriver/makefile brought enlightenment:
cp postgres.so /usr/local/aolserver/bin/
gave me the installation-page. yippi.
yes, it was "messy". i mean, i even modified sources and makefiles and such and restarted and killed various processes a thousand times. and it took forever. but really: the feeling is *much* more rewarding, i'll readily admit to that!
like the difference in enjoying the scenery when you climbed the mountain yourself or simply took the lift
so i guess, i'll be pestering this forum here soon with openacs-specific questions ;)