…to match the version that `superstring` expects.

This is copied directly from what Homebrew installs into `$(brew --prefix)/opt/libiconv`, except that the install name has been changed on `libiconv.2.dylib` to `libiconv.2.dylib`. (When it's installed from Homebrew, its macOS library “install name” matches its original path, so that's where macOS looks for it.

As part of the build, we rename the library with `install_name_tool` so that it can be resolved relative to the location of `superstring.node`.

This can probably be simplified, but first let's see if it'll work.

…on macOS.

…because we need a version built for the current machine's architecture.

(If our current approach doesn't work, we could vendorize two different copies — one for each architecture.)

…by grabbing it from Homebrew or from a path specified by the user in an environment variable.

…by adding an alternate strategy that we can adopt if we ever need to.

Also copy over `libiconv`’s README and license file.