My build settings have the OSX Base SDK Version set to 10.9 SDK. With JUCE 5.3.1 I get the build error:
'error: There is no SDK with the name or path ‘/Builds/MacOSX/macosx10.9’
It worked just fine with JUCE 5.3.0.
Setting everything to default (SDK and deployment target) also works with 5.3.1.