CMake/Mac only supports compiler features for AppleClang

On Mac, CMake only supports compiler features for GCC and AppleClang. This means that CMake will issue an error when configuring any project that uses Qt5 and a non-Apple Clang build. An incomprehensible error, at that.

Out-commenting the following line in Qt5CoreConfigExtras solves the issue:

set_property(TARGET Qt5::Core PROPERTY INTERFACE_COMPILE_FEATURES cxx_decltype)

NB: this is not about building Qt with a different compiler. This is about compiling 3rd party software, and that ought to be possible with the compiler of choice, esp. a compiler like clang that doesn't come with C++ runtime issues like GCC tends to.