c++11 giving error after "make" command in macOS (Big Sur) terminal :

I wanted to install a software (which requires c++11 or later) from github. I am using macOS Big Sur terminal. I have downloaded homebrew, xcode, eclipse IDE and all necessary git setup has also been done.I have installed llvm from brew and other similar suggestions already. After cloning the repository, I enter the said folder and give commands as follows:

cd seing
mkdir bin
cd src
make seing

After the last command I get the following error:

g++ -std=c++11 atom.cpp atomicsystem.cpp neighborlist.cpp fingerprintgenerator.cpp genericlocalcalculator.cpp gaussiancalculator.cpp inputs.cpp utilities.cpp bispectrumcalculator.cpp zernikecalculator.cpp main.cpp periodictable.cpp agnicalculator.cpp -o ../bin/seing -fopenmp
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
make: *** [seing] Error 1

I have tried all resources possible and cannot get through it. I have just started using git. Any suggestion?

Suggested: try adding -std=c++11 to your compile command (but i dont understand it)