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
infiniband - Enable Infiniband RDMA transport support
kernel_linux - KERNEL setting for system using the Linux kernel
libressl - Use dev-libs/libressl instead of dev-libs/openssl when applicable (see also the ssl useflag)