VS 6 warns verbosely when WINVER >= 0x0500. Avoid defining WINVER and
_WIN32_WINNT to higher than 0x0400 on VS 6. Provide missing API
declarations in archive_windows.h when we do not get them from
<windows.h>. Provide GetVolumePathNameW because VS 6 does not declare
it regardless of the API version.