The easiest way to verify this is to run
assuming that net-libs/libtirpc was present in the system when media-gfx/asymptote was built.
Also, you can see libtirpc detection in asymptote's configure.ac:
Also, I asked the author and he just added this library as an official requirement (in the upstream master repo):
and I don't expect that Gentoo will try to condition it with a USE flag (it seems possible, but not necessary, since libtirpc is a relatively basic library, as far as I can tell).
What to do with current versions of asymptote in Gentoo portage tree (2.61 and 2.62), I have no idea. I suppose that most asymptote installs have this unaccounted dependency…