https://app.assembla.com/wiki/show/portaudio/ReleaseNotes indicates that the latest release is version 19.6.0: 19.6.0 pa_stable_v190600_20161030 (October 30, 2016, Git 396fe4b669).
It's finally in the main overlay, but with the weird version number "19.06.00" instead of "19.6.0_pre20161030": https://gitweb.gentoo.org/repo/gentoo.git/commit/media-libs/portaudio?id=8457ba6f9fd6f0dbafcca1fb375788eeea02aae1
It looks like this made it into the tree a while ago.