cxx - Build support for C++ (bindings, extra libraries, code generation, ...)
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
ogg - Add support for the Ogg container format (commonly used by Vorbis, Theora and flac)
static-libs - Build static versions of dynamic libraries as well