Unable to build plug-in due to Xcode errors

Apparently out of the blue, I’m unable to build a plug-in using Xcode. I’m using JUCE 5.4.4, Xcode 10.1 (10b61), macOS 10.13.6. If I create an Xcode project using Projucer, Xcode complains: “-fobjc-weak is not supported on the current deployment target” It built fine last week!

After much fruitless messing around with build targets and other settings, I’ve made no progress at all. Any chance of a hint as to what might be the issue here?