CMake 3.15.5 bootstrap fail with g++ 9.3.0

I'm trying to build CMake 3.15.5 on a 20.04 xubuntu machine (but I also tried 18.04 and 20.10 with the same results).

The error is as follows:

loading initial cache file /home/max/Lavori/nova/src/repos/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Bootstrap.cmk/InitialCacheFlags.cmake
-- The C compiler identification is GNU 9.3.0
-- The CXX compiler identification is GNU 9.3.0
-- Check for working C compiler: /usr/bin/gcc
-- Check for working C compiler: /usr/bin/gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/g++
-- Check for working CXX compiler: /usr/bin/g++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Checking if compiler supports C11 _Thread_local
-- Checking if compiler supports C11 _Thread_local - yes
-- Checking if compiler supports needed C++17 constructs
-- Checking if compiler supports needed C++17 constructs - yes
-- Checking if compiler supports C++ make_unique
-- Checking if compiler supports C++ make_unique - no
-- Checking if compiler supports C++ unique_ptr
-- Checking if compiler supports C++ unique_ptr - no
CMake Error at CMakeLists.txt:92 (message):
  The C++ compiler does not support C++11 (e.g.  std::unique_ptr).


-- Configuring incomplete, errors occurred!
See also "/home/max/Lavori/nova/src/repos/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeOutput.log".
See also "/home/max/Lavori/nova/src/repos/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeError.log".
---------------------------------------------
Error when bootstrapping CMake:
Problem while running initial CMake

I am absolutely sure that g++ 9.3.0 supports C++11, so where could the problem be?