Gentoo's Bugzilla –
Attachment 446134 Details for
Bug 594072media-sound/klick-0.12.2 : src/metronome_map.hh:51:42: error: ‘constexpr’ needed for in-class initialization of static data member ‘const double MetronomeMap::TICKS_PER_BEAT’ of non-integral type [-fpermissive]