我在ubuntu 18.04中安装根（cern）时遇到问题

Question

我按照本页中所述的步骤操作：https://techforcurious.website/cern-root-installation-ubuntu-18-04-enabling-libraries/，但是当我放置cmake ../root-6.10.04/ -Dall=ON时出现下一个错误：

CMake Error at cmake/modules/SearchInstalledSoftware.cmake:825 (message):
  The Alien plugin requires option 'xrootd' to be enabled.  Re-run the
  configuration with 'xrootd=ON'

我使用xrootd=ON重新运行配置并显示：

 Set environment variable XRDSYS to point to your XROOTD installation
 Alternatively, you can also enable the option 'builtin_xrootd' to build XROOTD internally.

请帮助我。