Switch to libossp-uuid, to get UUIDs on Windows

This was only used to avoid a necessary check in collab builds on
Windows. @jtanx, who does the Windows binaries, says collab currently
isn’t building on Windows anyway.
It might also pay to make this version=3 in binary beacon and also add a check in the client code that if version > 3 then popup a message telling user to upgrade FontForge to join the session. That later part obviously can't be done retrospectively but adds some protection if we want to change the beacon version again in the future. I hadn't anticipated needing to do a mid data structure change any time soon, append only it would have given time to roll out this change in advance.