add notification in admin page about buggy libxml2 versions

libxml2 has caused us a range of issues - see: MDL-16759, MDL-17136, and I've just spent way more time than I should trying to debug an issue on a shared host environment that turned out to be another issue related to the libxml2 stuff - Moodle Networking (Mnet) doesn't work!

we should really add a notification based on the libxml version that gives admins the ability to resolve this issue. IMO adding stuff to faq/docs pages isn't really enough
Wordpress have used some code like this:
if(LIBXML_DOTTED_VERSION == '2.7.0? ||
LIBXML_DOTTED_VERSION == '2.7.1? ||
LIBXML_DOTTED_VERSION == '2.7.2? ||
LIBXML_DOTTED_VERSION == '2.7.3?)