MacOSX10.12.sdk

• If not done already, install MacPorts.
• To install MacOSX10.12.sdk, run the following command in macOS terminal (Applications->Utilities->Terminal)
  
  sudo port install MacOSX10.12.sdk Copy
• To see what files were installed by MacOSX10.12.sdk, run:
  
  port contents MacOSX10.12.sdk Copy
• To later upgrade MacOSX10.12.sdk, run:
  
  sudo port selfupdate && sudo port upgrade MacOSX10.12.sdk Copy

The MacPorts Project uses a system called Trac to file tickets to report bugs and enhancement requests. Though anyone may search Trac for tickets, you must have a GitHub account in order to login to Trac to create tickets.