It seems that the following two patches are needed: https://github.com/Kitware/CMake/commit/a997cac8323dc1764e092b8691857dd9af70956e https://github.com/Kitware/CMake/commit/9bb8ac8ed28811f6097233b1065b71833a017c0f Otherwise there are not Boost:: imported targets.
I've added =dev-util/cmake-3.7.0_rc1 which contains both commits. Like boost-1.62.0 this version has no keywords, so adjustment via package.accept_keywords is necessary.