OK, everyone, I fixed RTAS compilation on the newest tip, which surprisingly even works with the current Xcode 7.2 and OSX 10.11 El Capitan.
Here's a few infos:
You have to compile everything (the RTAS library itself as well as your JUCE plugin) with the following settings:
Base SDK: Mac OS X 10.6
OS X Deployment Target: OS X 10.6
C++ Language Dialect: C++98 [-std=c++98]
C++ Standard Library: libstdc++ (GNU C++ standard library)
For this you obviously need the 10.6 SDK, which you can download here and then put into /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs.
Unfortunately I could not succeed with compiling RTAS with any SDK newer than 10.6 because I could not get past the 'RegEntryID' compiler error described at the end of this post (if anyone knows how to get past this, please enlighten me)
You will also encounter a compilation error inside the RTAS SDK itself at this piece of code:
#ifdef __GNUC__ && __POWERPC__
#include <ppc_intrinsics.h> // needed for __eieio
The DigiDesign guys seem to have trouble with using the syntax of #if and #ifdef correctly ;-) Anyway, the fix is to just remove this bit of code altogether as we are obviously not on a PowerPC (and please don't ask for PowerPC support...)
Another catch is that our current AudioUnit wrapper does not compile with such an ancient SDK as OS X 10.6. So basically this means that you can't have a jucer project with AU and RTAS enabled simultaneously, as depending on what SDK you choose one of the two will always fail to compile.
I'd suggest that you keep a legacy jucer project with the SDK set to 10.6 if you need to compile old stuff with RTAS, and use another one with the current SDK for all new development.
FYI, we will do our best to maintain this way to get RTAS compiling (so as not to break your legacy stuff), but we will deprecate RTAS and we won't put any new development into RTAS (so for example new features like multibus won't be ported to RTAS). I also won't attempt to fix our AU wrapper to compile with the ancient OSX 10.6 SDK. We strongly advise you to give up RTAS and move to AAX instead. Many major audio software companies like e.g. Native Instruments have already done this a while ago.
Edit 02/02/2016: it turns out you can compile your plugin using C++11 and the newest OSX SDK as long as the PT_SDK library itself is compiled with the 10.6 SDK (see the rest of the thread below).