I just wanted to second pamputt’s comment about offline building. We have a similar situation at our lab with many computers on private networks where this will fail. I just wasted a LOT of time trying to figure this TBB issue out because the warning and error messages are completely inadequate. During the cmake stage, it only subtly says TBB is not found and it is switching to builtin_tbb. Nothing about requiring a network connection. Then, far into the build, I get an error which does point to some log file that finally indicates a download error. I go back to cmake and try turning off “builtin_tbb” which seems to be the source of the problem, but it won’t turn off and it doesn’t print any message indicating why. Without this forum post, I can’t imagine how I would know to turn off “imt”. (Nothing about that connection during the build or on the page https://root.cern.ch/building-root).
I’ve been building ROOT for years so was really caught off guard by the network requirement. I think adding the <4MB of TBB source to the 153MB is not so evil a burden for those with TBB already installed since I’m guessing it saves a lot more resources (of the human kind) to not have to track this issue down in the way pamputt and I did.