changes needed to implement XROOTD as external package together with
the alien and monalisa plugins. I have also introduced a new option
(builtin_xrootd) to build XROOD together with ROOT.


git-svn-id: http://root.cern.ch/svn/root/trunk@42100 27541ba8-7e3a-0410-8455-c3a389f83636