custom-cflags - Build with user-specified CFLAGS (unsupported)
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
gles2-only - Use GLES 2.0 (OpenGL for Embedded Systems) or later instead of full OpenGL (see also: gles2)