we desire libxml 2.8+ and as a side effect that also provide
thread-safe static in c++

Change-Id: I2a0f06a81c6a5734a8b3a859fb291e5ae80d12e9
Reviewed-on: https://gerrit.libreoffice.org/37818Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>