Can not build with clang 4.0 using C++14

In Qt 5.9.1, the definition of Q_REQUIRED_RESULT has changed in qcompilerdetection.h

 #if QT_HAS_CPP_ATTRIBUTE(nodiscard)         // P0188R1
 #  undef Q_REQUIRED_RESULT
 #  define Q_REQUIRED_RESULT [[nodiscard]]
 #endif

Compiling our code with gcc (version 6.3.1) with -std=c++ 14 this works fine but when using clang (version 4.0.0) with -std=c++ 14 we get

qcolor.h:194:5: error: use of the 'nodiscard' attribute is a C++1z extension
       [-Werror,-Wc++1z-extensions]

We're using C++ 14 with a compiler supporting C++ 1z (clang 4.0.0) but providing  -std=c++14. It seems like you ignore compiler arguments with the __has_cpp_attribute check.

define QT_HAS_CPP_ATTRIBUTE(x)       __has_cpp_attribute(x)

You can not force users to set std=c++1z when using a modern compiler.

We are using these cxx flags:

 -std=c++14 -pipe -pthread -Wall -Wextra -pedantic -Werror -pedantic-errors