Just a small correction. I use portage-2.0.54 so this may be fixed in later versions. cloud games-engines # man 5 ebuild | grep use_with myconf="$(use_with jpeg libjpeg)" myconf="$(use_with jpeg libjpeg)" myconf="$(use_with opengl") Note the last quote which should be after the paranthesis.
This is already fixed in portage 2.1