net-p2p/eiskaltdcpp-lib: Fix CMake version

Fix build with modern Boost:

• Boost::System is now header-only, so remove the find_package() component
• Since only header-only libraries are used, use plain find_package() call.

Remove the runtime dependency on Boost::Thread, because it is not used
by the library.

Add USES=iconv.

Event: Berlin Hackathon 202609