Gnu Emacs On AIX

[Karl Kleinpaste]

You probably just need to upgrade to a more recent incarnation of GNU
Emacs than 18.51.  The 18.55 etc/MACHINES says that 18.52 (at least)
copes:

IBM PS2 (m-ibmps2-aix.h; s-usg5-2-2.h)
  Changes mostly merged in 18.55.
  You may need to put an #ifndef AIX conditional 
  around the definition of closedir in sysdep.c.

IBM RT/PC (m-ibmrt.h or m-ibmrt-aix.h; s-bsd4-2.h or s-usg5-2-2.h)
  18.52 works on both operating systems.
  Use s-bsd-4-2.h for the 4.2-like system and s-usg5-2-2.h for AIX.
  On BSD, if you have trouble, try compiling with a different compiler.
  On AIX, the file /usr/lib/samples/hft/hftctl.c must be compiled into
  hftctl.o, with this result left in the src directory (hftctl.c is
  part of the standard AIX distribution).
  window.c must not be compiled with -O on AIX.

--Karl