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.
- 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
