librevenge states boost as RDEPEND+DEPEND (with slot op). However, it seems that boost is only used in a few .cpp files, via header-only libraries. The library doesn't link against boost, and its headers do not include boost's headers. Therefore, I think the dependency on boost should be moved to pure DEPEND.
commit 93ba66c4e59ce1a6c95130cb929300c4717f09af Author: David Seifert <soap@gentoo.org> Date: Sun Apr 16 20:19:33 2017 +0200 dev-libs/librevenge: Only DEPEND on dev-libs/boost Bug: https://bugs.gentoo.org/show_bug.cgi?id=607906