If you download the API Development Kit you can find every necessary file to build a General API based add-on.
The subfolder named Examples collects a rich set of example projects.
The subfolder named Support collects all the necessary libraries and header files. It includes also tools to create a new AddOn.
- Header Files
- Frameworks (only on macOS: this contains the binary frameworks of the modules)
The subfolder named Documentation contains the full documentation of the API and related software. It can be read either in HTML or CHM/docset format (depends on the platform).