I think,
Audacity and PortAudio teams take in mind MS Windows users as main
projects target, rather Linux users.
Actually I think most of the developers are using Linux or OS X
themselves. I think it's fairer to say that the Audacity team is
working on stabilising version 1.2.0, which is why PortAudio v19 is
only an experimental compile-time option rather than the default at
the moment. Testers for Jack support will probably be needed in the
near future though, so I do hope you will subscribe to the
audacity-devel list.
Cheers
Daniel