libiostream still exists only because gcc2.6.3 is still around
Have you installed g++ because it looks like this 'libiostream.a' is part of that package. If it isn't then I would suggest contacting the PDP++ package maintainers to ask where this dependency is coming from.
As far as passing flags to configure go, you can see what flags are availble by looking at your configure script. In my experience a short explanation about each flag is listed at the end of the file.
an example of a flag would be something like..