2.20.1 bugfix release

Hi Jonathan, as a followup to last year's 2.20.0 release, I just did a quick bugfix release in time for the upcoming Linux Audio Conference. This has a few important bugfixes, so it would be nice if you could merge this over here when you find the time. Thanks!

This release fixes numbox2 which inadvertently got broken in 2.20.0, and also backports some MIDI backend fixes from vanilla Pd which we didn't have yet. Most notably, the latter fixes at least one important bug in the PortMidi backend (related to the processing of realtime messages) which affected Mac and Windows systems.

And then there are the usual fixes to the build system and a bunch of gcc15 compilation fixes, so that the OBS and GH CI builds run fine again. While I was at it, I also removed the gconf2 dependency which is now obsolete, and was causing trouble on the latest Debian and Ubuntu systems where this package is no longer available.