Since this commit in master: https://projects.kde.org/projects/kde/kdeedu/kstars/repository/revisions/b8f5dc052a2dcc8107367af5921043be32eee0c3 KStars depends on eigen:3 to build and so configuration right now fails. Installing eigen:3 solves the issue. Reproducible: Always
Well spotted. Fixed in kde overlay. Please sync. http://git.overlays.gentoo.org/gitweb/?p=proj/kde.git;a=commit;h=ca7c8071a10b800018d652d03bded9f2304bf8f6