In short, it's not going to happen any time soon (previous FAQs referred to patches that a new NIHCL release would hopefully contain, but this hasn't happened).
[ From Steinar Bang <steinarb@idt.unit.no>]
InterViews 3.1 compiles and runs with gcc-2.3.3 and libg++-2.3, except that the "doc" application immediately dumps core when you try to run it. There is also a small glitch with idraw.
There is a patch for InterViews 3.1 from Johan Garpendahl <garp@isy.liu.se> available for FTP from site ``ugle.unit.no''. It is in the file
/pub/X11/contrib/InterViews/g++/3.1-beta3-patch.
This fixes two things: the Doc coredump, and the pattern menu of idraw. Read the instructions at the start of the file.
I think that as of version 2.5.6, the standard g++ will compile the standard 3.1 InterViews completely successfully. I'd appreciate a confirmation.