If I’m inferring correctly, JUCE is transitioning or has transitioned fully to C++11. (Might be mentioned officially somewhere, don’t remember!)
With that in mind, the condition to define JUCE_HAS_CONSTEXPR when compiling with Clang should be changed to #if __has_feature (cxx_constexpr) since cxx_relaxed_constexpr is C++14.
It may be a weaker version of the feature, but I’m trying to avoid any potential breakage with ABI compatibility with 3rd party static libs - something I have little control over, and haven’t the time to fight for a C++14 version of the libs.