preprocessor/4176: #pragma warnings are "invisible"

[Petter Reinholdtsen]

[neil@gcc.gnu.org]
>     Well, we are supposed to ignore warnings we don't understand.
>     To me, #warning is canonical with #error and makes more sense than
>     #pragma warning.  Adding this pragma would mean
>     yet another pragma in the global namespace.
>     I'm interested in what others, patricularly Zack, think
>     though.

I'm not asking you to add code to understand the pragma, just to add
some code to output the complete pragma string for unknown pragmas.