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.