Hi Lukas,
I’m afraid CentOS + devtoolset is not an officially supported platform, see e.g. the discussion here. In other words we do not test compilation of ROOT with devtoolsets, so it can break like this without us noticing.
Sorry about that! Feel free to open a bug report about the compilation failure itself.
There are a few options to get ROOT 6.22 on CentOS7, e.g. installing via conda.
Cheers,
Enrico