win32: use windowsappcompat instead of winstorecompat

This is the proper counterpart to windowsapp.

Now that we have a proper Docker image to build it:
registry.videolan.org/vlc-debian-llvm-uwp:20200603145315

A recent mingw64 8 (unreleased) is needed to make use of this. It's available
in our Docker images and in msys2 (although it's using msvcrt so it will
probably fail to link properly)

The forced -lwindowsappcompat is added like the other LDFLAGS in configure.ac.