http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50982
--- Comment #4 from David Edelsohn <dje at gcc dot gnu.org> 2011-11-03 18:11:40 UTC --- The failure is config/gthr-posix.h is not found in the search path when building libstdc++ during bootstrap. Paolo's suggestion probably was not well thought through. I tried editing gthr-aix.h to remove config/, but that was not sufficient and failed in another way. I will try with the other parts of your patch.
