Or rather, it can be built without it, but not depending on it can cause an
indeterministic build, with sometimes an iconv enabled libass, and
sometimes an non-iconv libass build.
On some other occasions, it can also fail:
- libiconv.a gets installed early on
- libass then determines that it can use it, regardless of iconv.h
- libass starts building
- ass.c iconv code fails to build due to missing iconv.h
- iconv.h gets installed afterward, but the build failed already