Hi,
You are using a version of ROOT that was built without implicit multithreading capabilities. If you are on lxplus, note that this feature is not available by default as described here . You could then use one of the LCG views to pick a version of ROOT with IMT, as suggested in this post.
If you still wish to build ROOT on your own, I advise you to take a look at the guide.
Cheers,
Vincenzo