Trying to build kicad against boost built with USE="-nls" results in an error that the boost_locale library could not be found. It doesn't appear that kicad has a configure option for disabling locales (ie, nothing that could be made into a nls useflag for kicad), so it looks like the boost dependency needs to be: >=dev-libs/boost-1.56[context,nls,threads,python?] With boost[nls], kicad emerges just fine.
Created attachment 415964 [details] build.log Report is spot on as per attached build log. Submitting fix. Thank you for the report.
Fix applied through github commit: https://github.com/gentoo/gentoo/commit/93ea091741c3342126b35fd234999e8c641b4721
