Since OpenSSL isn't used any more on OS X, there is no dependency
on any MacPorts library under /opt/local and there is no danger of
conflicts between MacPorts and system iconv. For this reason the
system iconv can always be used now.

As an alternative to OpenSSL when we're on OS X. This one can actually
take advantage of stacking the streams.

We don't really need our own module to find libssh2. Using pkg-config
lets the standard tool do the work for us and let us fit more naturally
in the workflow as we respect the pkg-config search paths.

So we actually also never know that we can set a dependency on it in
pkg-config. Instead always give it the -L and -l options.

- Add correct -I, -L and -l flags
- Search for libiconv in /opt/local/[include|lib] before in the
  system path. See #2017 for details.
- Give splitted -L and -l arguments to pkg-config

Doesn't change anything. Just removes stuff that was probably missed
to remove when this was imported.

* add FindIconv helper for CMake iconv detection
* only default using iconv to ON for MacOS
* update pkg-config generation to include iconv dependency better