I have rebuilt XFree86 4.2.x from sources tagged with
netbsd-1-6-RELEASE using the NetBSD 1.6 C compiler and it all works. 

Now all I need is to get the SVGA server to build, and to work out why
it wouldn't work with GCC 3.2. The latter could well have been caused
by me mixing and matching shared libraries from 4.1.0 and 4.2.x. It's
just one more thing for me to look at when I get around to caring enough.

Cheers,
Lloyd