The post is about building GCC itself with a C++ compiler, which is already possible on most(?) platforms: Basically, the default of the --enable-build-with-cxx

The post is about building GCC itself with a C++ compiler, which is already possible on most(?) platforms: Basically, the default of the --enable-build-with-cxx configuration option will change, which does not affect the default input language of the gcc executable (which is chosen by file extension anyway).