v6 of libconfig series

[David Bremner]

This obsoletes the remaining libconfig patches starting at

     id:1463135893-7471-6-git-send-email-david at tethera.net

I've addressed (my own) comments there. In 

[Patch v6 2/7] lib: config list iterators

I decided to use "placement new" to allocated a Xapian::TermIterator,
rather than a pointer.  This is slightly more exotic C++, but it
reduces number of dereferences in the code.

I think this series has got all the feedback it's going to, so I'm
going to go ahead and mark it ready to merge.