compiler detection fails with g++ on mac

qcompilerdetection.h

573 # if _cplusplus >= 201103L || defined(GXX_EXPERIMENTAL_CXX0X_)
..
617 # if __has_feature(cxx_generalized_initializers)

returns true because the compiler could support that feature when called with -std=c++0x (which qmake does), but the library does not (no header).
Please note that we have

#define _GXX_EXPERIMENTAL_CXX0X_ 1
#define __cplusplus 201103L
#define __cpp_attributes 200809
#define __cpp_constexpr 200704
#define __cpp_decltype 200707
#define __cpp_lambdas 200907
#define __cpp_raw_strings 200710
#define __cpp_rvalue_references 200610
#define __cpp_static_assert 200410
#define __cpp_unicode_characters 200704
#define __cpp_unicode_literals 200710
#define __cpp_user_defined_literals 200809
#define __cpp_variadic_templates 200704

We could detect this using (for example)

#define _GLIBCXX_ 20070719
#define _GNUC_GNU_INLINE_ 1
#define _GNUC_LIBSTD_MINOR_ 2
#define _GNUC_LIBSTD_ 4
#define _GNUC_MINOR_ 2
#define _GNUC_PATCHLEVEL_ 1

but the same happens probably is true for Q_COMPILER_RVALUE_REFS and probably other features, so I wonder if we should not disable basically all the __feature detection

The other question is also how much we want to support compiling with g+, for 5.3 it is still feasible (as we link libstdc+ in our packages), but 5.4 should link libc++ I think (and c++11).