That probably means you have vdt <= 0.4, please update vdt and try again. The vdt/tanh.h header is needed by TMVA and is only present in vdt 0.4 and above. There is already a JIRA issue for this problem if you want to track progress. I plan to add a better check for vdt in the next release of ROOT to catch this problem at configuration time.
Thanks for the answer, but the problem is not with external VDT. I’m trying to compile the standard root distribution with built-in VDT.
I studied the problem a bit and found out that it appears only in Mint 19.1 with the most recent updates (I don’t know exactly which updated package is guilty, maybe it was the Linux kernel headers or something like that).
I tried to compile root on another computer which had the same Linux Mint 19.1, but without updates. root-6.14.06 and root-6.16.00 were compiled without problems. After applying the updates root-14.06 still compiles, but root-6.16.00 produces the error which is in the title of the topic.