this should probably go to upstream, but here we are. with x11-libs/cairo[X] (which is quasi-default, as it is pulled in by x11-libs/gtk+:3, which has IUSE=+X), a binary cairo-sphinx gets installed - that is unusable as it is linked against libcairo-sphinx.so which gets installed in /usr/lib64/cairo where the binary can't find it. either drop install of cairo-sphinx, or add something like install_rpath: ${EPREFIX}/usr/$(get_libdir)/cairo to src/util/cairo-sphinx/meson.build
in 1.18.0, upstream has removed cairo-sphinx.