libxml2 on MacOSX

[Norbert Thiebaud]

as a follow-up to
it was necessary to use libxml2 as an internal library for MacOSX...
which was not well supported.

http://cgit.freedesktop.org/libreoffice/core/commit/?id=49843f0f9788e01805d8d0d27428fe5e6a4cc530

is what I had to do to get it to build. that included a change of
'layer' for libxml2...

I would appreciate some reviews as I am not very confident about the
implications...

Norbert