macOS mkmf.rb can't find header files for ruby

1. sudo rm -rf /Library/Developer/CommandLineTools
2. xcode-select –install
3. sudo xcodebuild -license accept
4. open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg

Ref: https://www.jianshu.com/p/a86e9b112191