When running e.g. Xcode 9.4 on macosx 10.12, the configure scripts fails with: "Failed to determine Xcode version" This is not very helpful.

Adding the output of the failed xcodebuild call (typically something like "Executable requires at least macOS 10.13, but is being run on macOS 10.12.6, and so is exiting") will help the user diagnose the error.