portaudio-18.1 doesn't set the soname-tag of libportaudio, which in turn in future could bring a dependancies breakage, because of (http://www.portaudio.com/docs/proposals/) API/ABI-change in the new release.
Created attachment 63184 [details, diff] portaudio-soname.patch This is a patch, which will set the soname-tag to libportaudio.so.0, must be applied before the ebuild copies Makefile.linux to Makefile.
The new media-libs/portaudio-18.1-r3 resolves this problem, so the bug could be closed as fixed.