Enlightenment will automatically enable composite support if it finds libXcomposite at the configure stage, however it won't be present unless the full xorg-x11 meta package (or another xcomposite supported package) is already installed. Also, as far as I can tell, all other packages with optional composite support already use this flag. I'll attach a simple patch against enlightenment-0.16.8.4.ebuild to add the xcomposite use flag.
Created attachment 101740 [details, diff] enlightenment-0.16.8.4.ebuild patch to add xcomposite use flag I've added x11-proto/compositeproto as a dependancy as well as x11-libs/libXcomposite, but I'm not sure how necessary that really is.
thanks, added to portage !