Crucially, the corrects the flags used for creating a DLL on Windows,
allowing threads.cmxs to be loaded in ocamlnat.

Necessary when building release tags.

If upgrading is forced and packages are required, combine the two calls.

ld -r (certainly in GNU binutils) has an empty search path - co-opt the
MSVC search code and always resolve libraries when partial linking,
except this time _ignore_ the ones which are missing. This seems to fit
the rest of -output-complete-obj, given that the _standard_ C libraries
are also omitted (-lm, -lpthread, etc.)