need finer-grained "C" header scheme information

[Benjamin Kosnik]

Kay. So, the "C" compatibility headers are working with my hacked
newlib.  Basically, I've used the --enable-cheaders=c configuration,
plus an additional layer of includes.

This new layer of "C" includes is include/c_compatibility. It just
injects std names into the global namespace, and is intended for the
C++ compiler only. 

The only issue remaining is represented by the following:

1) QNX
--enable-cheaders=c
(no need for c_compatibility headers as "C" headers already inject)

2) current newlib config
--enable-cheaders=c
need c_compatibility headers 

I suppose this is really just a configure question. I need to switch
parts of the --enable-cheaders=c model on and off based on target OS
info.

Thoughts on how to best do this?

-benjamin