Created attachment 419866 [details] emerge --info sys-devel/flex For whatever reason flex-2.6.0 seems to invoke its tests despite USE="-test" and fails due to missing sys-devel/bison (which is only pulled in for USE="test"). Emerging sys-devel/bison prior to merging is a functional workaround.
Created attachment 419868 [details] build.log
flex is running into a limitation of automake. BUILT_SOURCES cannot be made dynamic (like `make check`) and all of those are processed as soon as make is run in the subdir (regardless of target). http://gitweb.gentoo.org/repo/gentoo.git/commit/?id=8edfc50dab0424fe3db9741c3e5ccb9a0e0588a9