GNU version still not meets requirement after updating gcc/g++

[yntao-hust]

Previously I used ns-3.34 and my gcc/g++ version is 7, so in order to install ns-3.36, I need to update gcc/g++ version to at least 8.
Actually I have already installed gcc/g++ 8, and edit the best link version and current link point to gcc/g++-8, as follows.

and also, when I enter "gcc/g++ -v", it shows version 8.4

but when I build ns-3.36, it still shows: GNU 7.5.0 compiler is below the minimum required version 8.0.0, as follows:

Any ideas about this? thank you!