Actually import (i.e. register) the static plugin (not that it actually works, due to the annoying feature of "unused"
globals not being constructd :/)
Temporary hack hardcoded to get KatePart to be loaded.  Will need to generalise this inside QPluginLoader or somewhere like that.
Don't do this 'rpath' lunacy for Emscripten.
Use llvm-link instead of llvm-ar to produce static files - this enables us to add <LINK_LIBRARIES> to the non-native executable build flags, so all the required libraries are now statically compiled in (llvm-link - irritatingly - doesn't not know how to process archives - even those created by llvm-ar).  I've done the same for the native version, but will likely have to make it optional - see the comments on the corresponding emscripten-qt commit (1d2d2119f703eda24a2697ebfc6a47d466040e5f) for details.