[PATCH 17/19] Provide default definition for IS_IN when _LIBC is not defined

[Roland McGrath]

Please explain this more.  There must be on appearances of IS_IN or
anything like that in actual installed headers.  So I guess you are
talking about include/ wrappers for headers that will be installed.
But still I don't understand under what conditions any of this code
is used where _LIBC is not defined.  Is it just in tests?