We never search for libiconv via pkg-config

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.

@vmg Apparently it wasn't 100% reasonable. Because of the `IF(... MATCHES)` the pkg-config file was still broken. This fixes it, for real now :)

Btw.: This is true for OpenSSL, too. We set a dependency on it for pkg-config, but actually we cannot be sure that `FindOpenSSL.cmake` really found it via pkg-config...