fdo#51004: some progress and 10 years old bug pending

[Tor Lillqvist]

> There's traditionally been little
> effort in making any of those --with-system-* switches actually work on
> Windows,

This is true for "local" MSVC Windows builds, where --with-system for
various 3rd-party libraries indeed makes little sense.

But in the MinGW cross-compilation case, I think that we *do* intend
to have --with-system-* be meaningful, for contexts like the Open
Build Service (née openSUSE Build Service) where the "system" means
packages from the Build Service.

--tml