Yes, I have g++-multilib and all the other requirements installed. I just did an apt-get update; apt-get dist-upgrade, so I should have the latest of everything, with the same result. I'm wondering now if ccache or distcc may be interfering, so I'm trying a build without them.
Turns out something in my environment was causing the problem. Unsetting LIBRARY_PATH, CPATH and LD_LIBRARY_PATH (which only included $HOME/(include/lib)) got gcc building -- now on to everything else.
What are the ramifications of adding those to the environment variables unset by the Makefile?