Add iconv recipe (at least for Windows)

[micahsnyder]

Some projects, like libxml2, have an iconv dependency. On Windows iconv is not provided by the OS. It would be good to have iconv support in libxml2 and available for other projects too. Even clam uses iconv, if available, though clam can use the native win32 API for locale/utf8 conversion if iconv isn't present.