It seems that I am running XFree86 4.1.0, and not 4.2.0 like I thought. My
X install dates from early January this year.

I finally got a an X build that would run (using the cunning method of
not messing with new fangled C compilers) and I can't reproduce the
problem. I've tried building with and without -O2, and it just keeps
working. It's almost disappointing.

I'm now building a 'proper' copy of 4.2.x. Maybe I'll even get around
to looking at why GCC 3.2 won't build XFree86 properly. It fixes
various other problems I've encountered.

Cheers,
Lloyd