[RFC] "C" header options

[Benjamin Kosnik <bkoz at redhat dot com>]

The way I see it, the following options exist:

1) remove changed "C" signatures and builtins in string/wchar/math
   and have c_compatibility headers that just include_next<foo.h>

2) use changed signatures, builtins and figure out a way to make
   c_compatibility headers work for string/wchar/math
   seems fragile

3) remove "C" linkage requirement a la Stephen Webb's proposal
   which also removes inlining

4) remove compatibility requirement, and just use <cfoo> headers with
   this strange hybrid of zack's proposal from yesterday and a
   completely revised c_shadow. Strangely enough, this works if one
   simulates a PCH for the "C" headers (one include that includes all
   required "C" headers one after the other in _C_legacy and then the
   std_c*.h bits just include the PCH)

Ack. It looks like 1) is the only option that is at all robust.

I have sources that do all of these options, more or less. I'm going
to take a break for a bit, and see what others think.

-benjamin