I'm going to be a little picky and mention that libdir on windows should still be `$(BUILD)/lib`. However, `.dll` are classified as binaries not libraries. I abused the JL_LIBDIR name for the old setup, since I knew a priori all of its use cases. libuv has a special-case so that it got installed correctly (I see `libutf8proc.a` is now offending in this respect).

and, by this same trick, we avoid creating the lib folder and mostly avoid copying any `.a` files when creating the install