Error in building ROOT: std::atomic is not supported

[haltack]

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ROOT Version: root_v6.18.04
Platform: Scientific Linux 6.10
Compiler: GCC 9.3.0
CMake: CMake version 3.17.0

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Dear all,
I’m trying to build ROOT following the README.md:

git clone https://github.com/root-project/root.git
mkdir build
cd build
cmake ../root <- Here occurs an error!
make -j8
source bin/thisroot.sh
root

When I executed cmake ../root, I got the error message below:

CMake Error at interpreter/llvm/src/cmake/modules/CheckAtomic.cmake:49 (message):
  Host compiler must support std::atomic!
Call Stack (most recent call first):
  interpreter/llvm/src/cmake/config-ix.cmake:330 (include)
  interpreter/llvm/src/CMakeLists.txt:585 (include)

As yum provides outdated GCC and CMake, I built them myself from the source code.
I would appreciate if anyone helped me resolve this problem.
Thanks

[bellenot]

Maybe your gcc build is not complete, or you forgot libstdc++ std::atomic should be supported by gcc 9 and libstdc++ (see this page)