#include_next and removing of duplicate include dirs

[Benjamin Kosnik]

> Wouldn't it be simpler and easier in the long run to cooperate with
> the C library?

Yes.

> If they do (I believe this is the case for very recent Solaris,
> e.g.) we just need to link up with their mechanism for doing it.  If
> they don't, that's too bad, we don't claim perfect conformance on this
> target.

Right. Pending undiscovered cleverness.

> Certainly a header like <stddef.h> that is provided by GCC ought to
> have the necessary C++ compatibility support from the beginning, and
> need no wrapper from libstdc++.

Yep.

However. This is a low-priority item for me. In the meantime, should I 
remove the include_next from the c_std headers? Currently, it's not 
necessary and perhaps causing un-necessary problems.

Should I check in the
c_compatibility headers so that it's easier for people to understand what 
the final form for the "C" headers should look like? (They can be checked 
in, but not actively used). 

-benjamin