From Gerri:

Increase the flexibility for configuring with an external xrootd:
- add standard switches to disentangle lib and inc dirs
  --with-xrootd-incdir=<path_to dir_containing_XrdVersion.hh>
  --with-xrootd-libdir=<path_to_dir_containing_xrootd_plugins_and_libs>
- when passing a global xrootd dir with --with-xrootd, check both
  src/XrdVersion.hh and include/xrootd/XrdVersion.hh so that both
  build and install distributions are supported


git-svn-id: http://root.cern.ch/svn/root/trunk@31068 27541ba8-7e3a-0410-8455-c3a389f83636