sudo apt-get purge root-system* root-plugin* libroot*
(Note: afterwards check that the “/usr/lib/x86_64-linux-gnu/root5.34” and the “/usr/lib/i386-linux-gnu/root5.34” subdirectories disappeared completely.)
Then try to build your “new” ROOT version completely from scratch again.
See also:
[Installation problems (Ubuntu 14.04)
[Old version vs. New Version Installation Processes