linking problem of `master` for `AccumulatedVolumeMaterial` on Mac OsX

Hi,

master does not build on locally for me anymore

  Acts::AccumulatedVolumeMaterial::accumulate(Acts::Material const&)", referenced from:
      Acts::createMaterialGrid(std::__1::array<double, 3ul>, std::__1::array<double, 3ul>, std::__1::vector<std::__1::pair<Acts::Material, Eigen::Matrix<double, 3, 1, 0, 3, 1> >, std::__1::allocator<std::__1::pair<Acts::Material, Eigen::Matrix<double, 3, 1, 0, 3, 1> > > > const&, std::__1::function<std::__1::array<unsigned long, 2ul> (Eigen::Matrix<double, 3, 1, 0, 3, 1> const&, Acts::detail::Grid<Acts::AccumulatedVolumeMaterial, Acts::detail::Axis<(Acts::detail::AxisType)0, (Acts::detail::AxisBoundaryType)0>, Acts::detail::Axis<(Acts::detail::AxisType)0, (Acts::detail::AxisBoundaryType)0> > const&)> const&) in VolumeMaterialMapper.cpp.o
      Acts::createMaterialGrid(std::__1::array<double, 3ul>, std::__1::array<double, 3ul>, std::__1::array<double, 3ul>, std::__1::vector<std::__1::pair<Acts::Material, Eigen::Matrix<double, 3, 1, 0, 3, 1> >, std::__1::allocator<std::__1::pair<Acts::Material, Eigen::Matrix<double, 3, 1, 0, 3, 1> > > > const&, std::__1::function<std::__1::array<unsigned long, 3ul> (Eigen::Matrix<double, 3, 1, 0, 3, 1> const&, Acts::detail::Grid<Acts::AccumulatedVolumeMaterial, Acts::detail::Axis<(Acts::detail::AxisType)0, (Acts::detail::AxisBoundaryType)0>, Acts::detail::Axis<(Acts::detail::AxisType)0, (Acts::detail::AxisBoundaryType)0>, Acts::detail::Axis<(Acts::detail::AxisType)0, (Acts::detail::AxisBoundaryType)0> > const&)> const&) in VolumeMaterialMapper.cpp.o
  "Acts::AccumulatedVolumeMaterial::average()", referenced from:
      Acts::createMaterialGrid(std::__1::array<double, 3ul>, std::__1::array<double, 3ul>, (Acts::detail::AxisBoundaryType)0>, Acts::detail::Axis<(Acts::detail::AxisType)0, (Acts::detail::AxisBoundaryType)0>, Acts::detail::Axis<(Acts::detail::AxisType)0, (Acts::detail::AxisBoundaryType)0> > const&)> const&) in VolumeMaterialMapper.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
[99/269] Building CXX object Tests/Core/Fitter/