multilib-portage relies on the assumption that forcing multilib useflag off will suppress any mutilib support in the ebuild. The ABI-looping that multilib-portage does will interfere with most ebuild multilib build systems. Since libtxc_dxtn has no dependencies to speak off, the multilib use flag wasn't added initially. I'll attach a patch that adds the multilib USE flag and logic without prodding too much at an already elegant ebuild.
Created attachment 321208 [details, diff] add multilib USE flag and logic
Good idea. Applied and committed. Thanks!