This directory contains third-party projects that maybe be needed to
build Spot or execute some of its test cases, and that are
automatically downloaded in some of our scripts.

The main reason is to keep a local copy of them is to avoid download
issues (e.g. SourceForge has an impressive record of SSL and downtime
issues).  A nice side-effect is that is reduces network traffic.

See update.sh if you want to need where those files are copied from.