Building root with standard set to C++20 results in compilation errors on Linux

@jonas, should I also file a ticket for the warning I’m getting during compilation:

In file included from /home/jade/root/linux-build/include/ROOT/RConfig.hxx:23:
/home/jade/root/linux-build/include/RConfigure.h:30:4: warning: "The C++ standard in this build does not match ROOT configuration (201709L); this might cause unexpected issues" [-W#warnings]

This complaint comes up several times during the build. The build succeeds, but I don’t know how this warning will impact root’s performance.

PS Improvements to ROOT’s CMake configurations has been posted.