CONFIG += c++11 does not compile with clang on Mac OS X

CONFIG = c+11 does not work with clang on Mac OS X

Reason: #include <initializer_list> in qlist.h
initializer_list is not found, because the standard mkpsec for clang does
not use libc++ (-stdlib=libc++).

Since there are mkspecs for libc++ I guess Q_COMPILER_INITIALIZER_LISTS should not be defined in this case.