libc++ Compatibility on OS X

We are looking into migrating our C++ usage on Mac to libc++ instead of the deprecated libstdc++. This is a top-down Adobe thing, but is also the right thing to do.

Unfortunately, we've found some potential incompatibility issues that may pop up in third-party AMIO plug-ins (I don't think anyone has written a Session plug-in) and other plug-in formats should be unaffected.

If you've released an Audition plug-in on Mac before, please reach out to us.