debug - Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
gtk - Add support for x11-libs/gtk+ (The GIMP Toolkit)
openmp - Build support for the OpenMP (support parallel computing), requires >=sys-devel/gcc-4.2 built with USE="openmp"
telemetry - Send anonymized usage information to upstream so they can better understand our users
vulkan - Add support for 3D graphics and computing via the Vulkan cross-platform API
