Changing default cxx_stdlib to libc++

[Joshua Root]

Code to check C++ stdlib linkage in rev-upgrade is now in master. That
takes care of the main obstacle to being able to change the default stdlib.

That leaves a couple of questions:

Which OS versions can we make the change on? At first glance it looks
like probably 10.6 through 10.8. 10.5 appears to have a much more
involved bootstrap process.

Do we need to make changes to the compiler selection logic? The
LibcxxOnOlderSystems instructions override the selection in
macports.conf. Will there be any bootstrap issues for users after the
switch?

- Josh