probable bug in powerpc-*-aix*, rs6000-*-aix* configury

[Nathanael Nerode <neroden at doctormoo dot dyndns dot org>]

While working on cleaning up top level configure.in, I uncovered a
probable bug in the configuration of rs6000-*-aix* and powerpc-*-aix*.
I'd like someone who's worked with one of those ports to verify this for
me before I fix it.

The issue is that these configuations set "use_gnu_ld" to 'yes'.
"use_gnu_ld", despite its name, controls only one thing: if there was no
--with-gnu-ld or --without-gnu-ld option specified, and we are building
ld in the source tree, should we default to --with-gnu-ld?

If use_gnu_ld is *blank*, the defaulting is done; if it is non-blank
(normally 'no') the defaulting is not done.

So 'yes' has the effect of 'no'.

If we want this defaulting on AIX, the lines should be removed.  If the
current behavior is correct and we *don't* want the defaulting, I'm
going to change them to "=no" for clarity, since the current lines are
misleading.

Thanks in advance...

--Nathanael
neroden@twcny.rr.com