if USE flags are exclusive/competing, then the best fit should be selected according to upstream and a warning issued, rather than aborting for example, if none of agg/cairo/opengl is enabled, then a warning should be issued that agg is automatically selected if more than one of agg/cairo/opengl is enabled, then a warning should be issued that the order of preference is agg > opengl > cairo and the build should continue the same goes for gstreamer/ffmpeg ... select one for the user if none are selected and give preference according to upstream defaults if both are selected
I agree, it's very common to have both cairo and opengl use flag enable, I don't know which one should selected for gnash in that case, but one has definitely to.
ive added 0.8.1-r1 that does this