> Do you know what glib++ does? There are some (cryptic?) remarks in the
> install notes about changing vendor files! That sounded very risky to
> me, so I wanted to install the library in a separate location...  but
> I could not easily deduce if this was possible and I was not ready to
> just take the chance that nothing would happen. At which point I gave
> up. :-(

glib++ is a set of C++ classes, designed to make your life much easier.
Keep them, and use them!

GCC shouldn't modify any vendor header files - they've already been
made C++-aware. I believe the installation process knows this; it
didn't change any of my headers.

If you're very paranoid, you could install gcc into /tmp (by changing
the Makefile) as an unpriviledged user.  You could then manually move
everything into the /usr/local hierarchy as root.