These are imported 3rd party library sources, wrangled for ease of plonking into
the build (e.g. relative #includes, etc.).

Used in SST itself:
  - monocypher

Used in debug builds, but not compiled into releases:
  - udis86

Most of the C sources have wrappers in the parent directory to build proper
objects for use in the project, and wrapper headers to get the full APIs as
conveniently as possible. In other words, most of these files aren't built or
used directly.

It is possible that these libraries may end up lightly modified; we err on the
side of changing things to fit our use case rather than working around problems
in outer layers. Sometimes libraries don't see updates for a while, other times
we don't bother updating them even when they do. Generally all this stuff is
stuff we're happy to maintain in-tree, and often stuff we don't need to update
much either because it already works fine.

IMPORTANT! Libraries are distributed subject to their copyright notices; please
refer to those!