JACK2 v1.9.21 release

A new version of JACK2 has just been released.
You can grab the latest release source code at https://github.com/jackaudio/jack2/releases and macOS/Windows installers at https://github.com/jackaudio/jack2-releases/releases.

This release focuses on bug fixes, particularly interesting for users might be macOS 12 compatibility.

The official JACK2 changelog is:

Add shell mode to jack_control (executes commands from stdin until EOF)
The waf autooption --example-tools introduced in 1.9.20 is now off by default, To get the previous behavior back pass --example-tools during build. This flag (and the related tools and their code) are going to be removed in the next release.
Fix alignas() on non-packed architectures
Fix build of jack-example-tools man pages (1.9.20 regression)
Fix compatibility with macOS 12
Fix missing symbols from jack client library (error and info callback pointers)
Fix potential memory corruption in midi_latency_test tool
Fix JackWeakAPI on Windows
Use predefined variables in pkg-config file (required for mingw)

External changes, related to macOS/Windows installer: