AIX incorrectly detects strndup

[Matthew Woehlke]

...for some reason, configure on AIX is incorrectly thinking that it has 
strndup (declared, anyway) from the OS, when it does not UNLESS 
'_ALL_SOURCE' is defined. This causes a build failure in xstrndup (from 
coreutils 6.6, and Jim's 6.7+ snapshot from Jan-26).

I know string.h has been seeing a lot of love recently, does this sound 
like something already fixed (or more likely, irrelevant?) or should I 
investigate why configure's check is failing?

--
Matthew
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