emacs 21.2 fail to compile on AIX 5.1

[Petter Reinholdtsen]

emacs 21.2 fail to compile on AIX 5.1, because config.guess returns
'rs6000-ibm-aix', and this is taken as an indication that this
platform is AIX 3, where AIXHFT is defined.  Defining AIXHFT triggers
inclusion of <sys/hft.h>, and this file is missing on AIX 5, as it was
missing on AIX 4.

I'm not quite sure how this should be solved, but I believe upgrading
config.guess to report the version number of AIX, and then fixing
configure.in to treat aix 5 as aix 4.* is a good way to start.

Newer config.guess outputs the following on our machine:
powerpc-ibm-aix5.1.0.0