Why does MacPorts says "this installation is configured to use libc++"?
Because MacPorts installations on Mac OS X 10.6 and later are configured by default to use libc++. The setting is cxx_stdlib. It can be changed in macports.conf. The setting does not appear in the default macports.conf because it is not really intended to be changed, and if you change it you must uninstall and reinstall all C++-using ports.
What should be done to avoid this error?
The port should not compile C++ software using g++. If that cannot be avoided, and if the port does not use any C++ libraries installed by another port, and if the port does not install any C++ libraries that are to be used by other ports, then the port can suppress the warning by setting (IIRC) configure.cxx_stdlib macports-libstdc++.
