following mods:

  - without arguments installs xrootd-<vers-num> in the current directory.
  - build directory will be deleted by default, unless option -k or --keep
    is specified.


git-svn-id: http://root.cern.ch/svn/root/trunk@42034 27541ba8-7e3a-0410-8455-c3a389f83636