This problem with Carthage.
"xcrun: error: unable to find utility "xcodebuild", not a developer tool or in PATH"
Setting the Command Line Tools in Xcode preference. Go to:
Xcode > Preferences > Locations
This can happen even if you have only one version of Xcode . It happens when you instal Homebrew before the first Xcode installation (which makes sense because you may want to install Xcode through mas install 497799835 and mas needs homebrew).
sudo xcode-select -s /Applications/Xcode.app/Contents/Developer
sudo xcode-select -s /Applications/Xcode.app/Contents/Developer; sudo xcode-select -s /Library/Developer/CommandLineTools