If the Ubuntu system provided “libclang-dev” package interferes with ROOT 6 then this is a serious problem (this may mean that ROOT 6 took “clang” instead of “gcc”, at least for part of its installation, and maybe it also tried to use any Ubuntu’s “cling”, if you have it installed).
Try to execute:
root-config --cxx
and see if you get “g++”.
There is a similar mysterious problem reported here: ROOT build suddenly failing (Ubuntu 16.04 LTS)
Maybe you could try to run (report here what is found):
sudo dpkg --list “*”|grep -i “cl[ai]ng”