Fix linking error under Win 7 (MinCore_Downlevel.lib cannot be imported for Win 7 or earlier)

Because binaries that link to MinCore.lib or MinCore_Downlevel.lib are not designed to work on Windows 7, Windows Server 2008 R2 or earlier (http://msdn.microsoft.com/en-us/library/windows/desktop/hh802935%28v=vs.85%29.aspx) so MinCore_Downlevel.lib can be imported only if target is Windows 8 or newer.