setting g++ mode to C++11

I am trying to build cmake source, which requires C++11. The build halts and apparently the complaint is that C++11 is not detected. The g++ mode is actually set to -std=gnu++17

From looking around on the web, I noticed that C++11 is only available after gcc version 4.6. I checked my version, and it seems to be above. g++ --version g++ (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609

I understand the -std=c++11 flag is used to enable the C++11 features in g++, but I don't seem to know what I am doing in this case.

I came upon this page which points to the cmake source I am using. Not sure that's any help though.

How exactly do I change the g++ mode, to C++11, with the bootstrap build of cmake?