%%% src_config
Option \Opt{-nostdinc++} is added to \Envar{CXXFLAGS}. That is due to bug:

\URL{https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100017}

Original bugreport describes the problem during the build of the whole \Prog{gcc},
but here it reproduced for standalone \Prog{libstdc++}.
