Martin pointed out that the new settings are not correct if the external
xrootd is passed via "--with-xrootd=<dir>" instead of a full specification
of inc and lib dirs via "--with-xrootd-incdir=<incdir>"
and "--with-xrootd-libdir=<libdir>".


git-svn-id: http://root.cern.ch/svn/root/trunk@35833 27541ba8-7e3a-0410-8455-c3a389f83636