I tried building Emacs pretest 24.3.91 on an AIX 7.1 POWER host and got 
the same problem.  The attached patch fixed things for me, so I 
installed it as emacs-24 bzr 117140.  Could you please try the attached 
patch on your AIX host, in 32-bit mode?  Please patch just 
src/unexaix.c, as the patch to 'configure.ac' is likely to cause you 
more trouble than it's worth if you're trying to build on AIX 6.  Thanks.

PS.  As far as I know Emacs has never worked on 64-bit AIX, so I didn't 
look at getting that working.