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