Can't build Emacs 21.3 on AIX 5.1

[gwinn@uk.ibm.com]

I am trying to build Emacs 21.3 on AIX 5.1 and it fails fairly early on 
in the make with errors relating to header files. I noticed in the 
output from configure that it correctly identifies the platform as 
powerpc-ibm-aix5.1.0.0 but a few lines further on I see

checking for AIX... no

This is odd, so I looked in the configure script and it prints that line 
when it finds that confdefs.h is missing. In fact my system does not 
have that header file (so configure is partly right). This makes me 
wonder if the O/S isn't installed completely or correctly or that 
something isn't right with header files. Has anyone seen this before? 
What is the confdefs test trying to do?

Thanks in advance.

Regards,

Geoff.