How to install ROOT 6.08.06 using gcc (version 8.3) different than the default gcc (version 4.8)

First, you should use CMake instead of "configure"as written here. Maybe, it will help you to solve this issue. I cannot say too much because usually I only use one compiler version to avoid compiling issue. So you should add the path to your gcc8 in your $PATH. The same for $LD_LIBRARY_PATH and try to compile again.