backward -> deprecated

[Benjamin Kosnik]

Nathan, I guess you are right. All the junk that's in there is a bit
discouranging to me though... I agree, something must be done.

I mean, we still have old hp header names in backward. UGH.

I wish the standard actually specified all the required headers, and
clearly indicated that some of the "more-or-less tradional" headers
were on the outs...

Looking at the pieces of a final solution:

Phil, Alexandre, and Jason have all indicated some kind of warning for
using the backward headers, unless a specific (already existing)
warning flag is used. To me, this means some kind of macro definition
triggered via this flag (done in cpp a la _EXCEPTIONS)

Then, we could add a third directory to the g++ include search patch
(a la the target include work) for backward headers, search it, and
install the contents of the backward directory into it.

Sound reasonable? Other approaches that I've missed that would be easier?

Neil?

-benjamin