message: Update the mac_sdk to 12.1
diff: 'compiler_host_version_int%': '0', # (major_ver) * 100 + (minor_ver)
  # Versioning stuff for Mac.
  - 'mac_sdk%': '10.15',
  + 'mac_sdk%': '12.1',
  'mac_deployment_target%': '10.9',
  # Flag to specify if the build target is for simulator or not.
  