Milter-greylist

> > configure.ac:94: error: possibly undefined macro: _AC_DEFINE_UNQUOTED
> >       If this token and others are legitimate, please use m4_pattern_allow.
> >       See the Autoconf documentation.
> 
> Do you have it defined in a file there?
> grep -r _AC_DEFINE_UNQUOTED /usr/share/autoconf/autoconf/
> 

No, I do not have if defined.

My autoconf is actually the latest in the CentOS 6 production release. Version information:

autoconf (GNU Autoconf) 2.59
Written by David J. MacKenzie and Akim Demaille.

Copyright (C) 2003 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.