Just to expand on what Dan has added above, the header files are contained in the source dist. of aolserver, note that they are not distributed in the binary distribution of aolserver.
The easiest way I have found of compiling the driver is to first extract the aolserver source file into the ~nsadmin directory, next copy the driver-2.3 directory into ~nsadmin, next edit the makefile under driver-2.3 to reflect the location of your postgres libs and aolserver home directory. Once you have saved the changes, type make, which ultimately should produce a file postgres.so which I then copy off to a temp directory. Then clean out all files from ~nsadmin and extract the binary aolserver distribution, making a copy of postgres.so that is in your temp directory and placing it in ~nsadmin/bin. Finally configure your nsd.tcl to load the driver...
HTH
