[v3] install backwards headers in prefix

[Benjamin Kosnik]

> #ifndef _CPP_BACKWARD_DEPRECATED_HEADER_WARNING
> #define _CPP_BACKWARD_DEPRECATED_HEADER_WARNING
> #warning "this file includes at least one deprecated header,"
> #warning "(in general) use <X> form instead of <X.h> form"
> #endif

yes. Patches accepted. I believe the consensus is to actually emit the 
warning unconditionally, so the macro bits aren't necessary, just the 
#warning flags.

-benjamin