make sure to first try to get /usr/lib/libxml2 before trying /sw/lib/libxml2

on MacOS X. The /usr/lib version is a proper universal binary containing a 
64-bit version.


git-svn-id: http://root.cern.ch/svn/root/trunk@23636 27541ba8-7e3a-0410-8455-c3a389f83636