This contains third party libraries that are uncommon and are therefore shipped
with the Widelands source code for transparent compilation.

Changes
-------

Eris:
- removed luac.c. It defined a main() function which could hide our own main()
  eris was linked in.

libmd:
- Based of commit d5b8e853989a73f8fff9dc4e00dccd0b691b84f9 (2024-02-21);
  heavily customised to embed it into our portable codebase and avoid name collisions with various system headers and libraries.
- Converted all files to C++, moved them into the "libmd" namespace, and renamed the include guards.
- Included some of the files that are normally generated semi-automatically.
- Removed file input functions specific to POSIX.
- Removed support for MD2, MD4, and RipeMD160.
