The documentation that is built in doc/ depends on imagemagick's convert(1) to convert eps files into png.For that, imagemagick with USE=png needs to be available. However, imagemagick is not currently a dependency of asymptote, resulting in a broken build (if imagemagick is not present) and a bunch of warnings as well as partly broken documentation if USE=png wasn't specified.
Thanks for the report, assigning to maintainer
Thanks, I've added this dependence. Will commit on Monday.
asymptote-1.43-r1 is in cvs. Thanks.