pidgin, *: remove ugly win32 pidgin hacks, add Nix expression (!), fix utf8cpp

- The ugly DLL loading wrapping bollocks in the pidgin backend has
  been ripped out. Yay no more wrappers!
- The .gitignore blacklisted include/transport/utf8/core.h, which
  is kinda required...
- We now have a Nix expression to build the whole thing, which is
  very helpful.
- Also, some cmake stuff was tidied up.