If we're building static libraries, we want to use that for building our clar
binary. This is done in 49551254 (2017-09-22; cmake: use static dependencies
when building static libgit2) but that commit included the rule too early,
making it affect the search for iconv, meaning we did not find it when we were
building a static libgit2.

Move the rule to just before building clar, after we've included the rules for
building the library itself. This lets us find and link to the dynamic libiconv.