That is quite strange, but still probably has nothing to do with single core vs multi core. Either the library is not found at the path listed in the error message, or it is found, but is incompatible (different version that what rootcling requires). Looking again at the error message, I see that this is not about glibc, but about libstdc++, so you probably have a mismatch between GCC and clang. How are you configuring ROOT with CMake? What compiler are you using? You may be using a compiler from outside the anaconda installation and that compiler is not compatible with the libraries installed there. If you can, please move to using the master branch, as you could be hitting some compatibility problems that were solved by the latest LLVM upgrade, as mentioned in this other post.