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).
(actually I want to build host-cmake in buildroot 2020.02.1)

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?

Here is the CMakeError.log file:

Determining if compiler supports C++ make_unique failed with the following output:
Change Dir: /home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_f5a77/fast && make[2]: Entering directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[2]: Warning: File 'Makefile' has modification time 0,27 s in the future
make -f CMakeFiles/cmTC_f5a77.dir/build.make CMakeFiles/cmTC_f5a77.dir/build
make[3]: Entering directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[3]: Warning: File 'CMakeFiles/cmTC_f5a77.dir/flags.make' has modification time 0,26 s in the future
Building CXX object CMakeFiles/cmTC_f5a77.dir/cm_cxx_make_unique.cxx.o
/usr/bin/g++    -O2    -std=gnu++17 -o CMakeFiles/cmTC_f5a77.dir/cm_cxx_make_unique.cxx.o -c /home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Source/Checks/cm_cxx_make_unique.cxx
Linking CXX executable cmTC_f5a77
/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Bootstrap.cmk/cmake -E cmake_link_script CMakeFiles/cmTC_f5a77.dir/link.txt --verbose=1
/usr/bin/g++  -O2   -L/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib -Wl,-rpath,/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib  CMakeFiles/cmTC_f5a77.dir/cm_cxx_make_unique.cxx.o  -o cmTC_f5a77 
make[3]: warning:  Clock skew detected.  Your build may be incomplete.
make[3]: Leaving directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[2]: warning:  Clock skew detected.  Your build may be incomplete.
make[2]: Leaving directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'


Determining if compiler supports C++ unique_ptr failed with the following output:
Change Dir: /home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_02c6e/fast && make[2]: Entering directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[2]: Warning: File 'Makefile' has modification time 0,27 s in the future
make -f CMakeFiles/cmTC_02c6e.dir/build.make CMakeFiles/cmTC_02c6e.dir/build
make[3]: Entering directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[3]: Warning: File 'CMakeFiles/cmTC_02c6e.dir/flags.make' has modification time 0,26 s in the future
Building CXX object CMakeFiles/cmTC_02c6e.dir/cm_cxx_unique_ptr.cxx.o
/usr/bin/g++    -O2    -std=gnu++17 -o CMakeFiles/cmTC_02c6e.dir/cm_cxx_unique_ptr.cxx.o -c /home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Source/Checks/cm_cxx_unique_ptr.cxx
Linking CXX executable cmTC_02c6e
/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Bootstrap.cmk/cmake -E cmake_link_script CMakeFiles/cmTC_02c6e.dir/link.txt --verbose=1
/usr/bin/g++  -O2   -L/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib -Wl,-rpath,/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib  CMakeFiles/cmTC_02c6e.dir/cm_cxx_unique_ptr.cxx.o  -o cmTC_02c6e 
make[3]: warning:  Clock skew detected.  Your build may be incomplete.
make[3]: Leaving directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'
make[2]: warning:  Clock skew detected.  Your build may be incomplete.
make[2]: Leaving directory '/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/CMakeFiles/CMakeTmp'

I cannot figure out wath is wrong.
I thought it could be a clock skew problem but I tried also outside a virtual machine with identical results.

I also tried to compile and link by hand (using the command lines above) without any errors or warnings.

/usr/bin/g++    -O2    -std=gnu++17 -o cm_cxx_make_unique.cxx.o -c /home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/build/host-cmake-3.15.5/Source/Checks/cm_cxx_make_unique.cxx

/usr/bin/g++  -O2   -L/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib -Wl,-rpath,/home/max/Lavori/nova/srcs/repo/dep-startup/buildroot-output/buildroot-2020.02.1/output/host/lib  cm_cxx_make_unique.cxx.o  -o cmTC_f5a77 

I also tried to run the binary without errors.
So even more 'I can not figure out what is wrong

best regards
Max

The clock skew warning is probably causing this code to assume that the feature is unavailable.