Default ACLs for FILE: ccaches on Windows

[Sam Hartman]

I am concerned about the complexity of adding this sort of
platform-specific functionality.  We've tried very hard over the years
to remove rather than add platform-specific functionality to the file
ccache.  I don't want to end up supporting ACL APIs for every
platform.

I don't see this item as urgent, so I'd prefer to allow it to be
blocked by a general policy on what platform-specific extensions we
accept and how we do that.

--Sam