The Mac OS X final release includes two files in the /usr/libexec directory.
config.guess
config.sub
copy these to your postgres source location then run configure.
These files include the info for the configure file to recognize the host.
There are also issues with shared libaries and you may need the libtool patch.
For more info in compiling source on Mac OS X see [fink.sourceforge.net/darwin/index.php