[Development] Compiler errors with clang 3.1

[Stephen Chu]

<https://bugreports.qt-project.org/browse/QTBUG-25817>

I tried both MacPort clang 3.1 and Apple's so-called LLVM 4.0 to build 
Qt and create apps with C++11 turned on. Both failed with constexpr errors.

On the other hand, compiling using gcc 4.7 is successful. I am not sure 
if gcc is more lex on this or Q_DECL_CONSTEXPR macro is not defined.