Ensure C++11 support matches between Qt and user projects for static builds

Change-Id: Id529fb7fc52d2da312bcf17612e47c74939a617f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Explicitly use libstdc++ for non-C++11 static builds

Otherwise the compiler may choose libc++ based on the deployment target,
and we'll end up with broken builds due to the mismatch between the two
libraries, eg:

Undefined symbols for architecture x86_64:
  "std::ios_base::Init::Init()", referenced from:
      __GLOBAL__I_a in libQt5Qml.a(qv4object.o)
      ...
  "std::ios_base::Init::~Init()", referenced from:
      __GLOBAL__I_a in libQt5Qml.a(qv4object.o)
      ...
  "std::__throw_length_error(char const*)", referenced from:
      ...

This problem is not iOS specific, which is why the logic is moved
to the more generic mac/default_post.prf.

Change-Id: I28b94e614f9167fc0db84bbf1c88dd97d5629938
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>