2.5 and cxx_stdlib

[Joshua Root]

Just a reminder that MacPorts 2.5 will check whether ports are built
against the right C++ standard library as part of rev-upgrade. The
"right" one is whatever configure.cxx_stdlib is set to for each port,
which will have a default value chosen based on the OS version (or
macports.conf) if the portfile does not set it explicitly.

This is very likely to give rise to bug reports against ports that don't
use the one they say they do, so be prepared to make some fixes.

- Josh