Problem resolved!
Davilla - Many thanks for your time last night. I'd still be hopelessly stuck if you hadn't narrowed the problem down to the 'configure rsxs-0.9' command or suggested that something might be amiss with my X11 install.
Here's the resolution for the benefit of anyone who encouters similar problems in the future...
I nipped home at lunchtime and tried running the 'configure' command from the native box, with local keyboard, mouse and LCD TV, in case there were problems detecting the OpenGL stuff when accessed via SSH without a powered-up monitor/TV. This made no difference whatsover, so I thought I'd fire up the X11 application to see if it would fail or not.
I couldn't find X11 in my Applications/Utilities folder (where I'd expect to find it on my old 10.4 PPC machine) and wasn't really sure if there's supposed to be an X11 application in 10.5 or if the underlying libraries only spring into action if you launch an application that requires X11
.
I then thought I'd try to reinstall X11 from my OS X install disks, but the disk is on loan to a mate who damaged his own
and is off on holiday for another week
.
Finally, I found X11 2.3.2.1 available as an install package at
http://xquartz.macosforge.org/trac/wiki/X112.3.2.1 and nervously installed it
.
The configure command now works properly
and I've left the machine running through the rest of the build process whilst I'm at work. If everything looks fine when I get back home this evening, I'll try to build from the coreaudio-osx branch and see whether I can get AC3 audio working on my old Sony amp the way my trusty old Xbox used to do!