- Recent updates of the msys2 dlfcn package don't allow libdl to be
  linked dynamically any more, which breaks Gem's automake+libtool
  build. (As it detects dlopen, it tries to link against libdl, which
  means that it can only be linked statically, which doesn't work in
  Purr Data.) To fix this, we artifically make the configure check for
  -ldl fail, so that it falls back to native Windows LoadLibrary et al.

- The proprietary plugin modules recently added to Gem (AVT, DeckLink,
  etc.) fail to compile on mingw, so we disable them for now.