Instead of forcing iconv support on macOS (by forcing `USE_ICONV`
on), honor the `USE_ICONV` option only on macOS.

Although macOS includes iconv by default, some macOS users may have a
deficient installation for some reason and they should be provided a
workaround to use libgit2 even in this situation.

iconv support is now disabled entirely on non-macOS platforms.  No other
platform supports core.precomposeunicode, and iconv should never be
linked.