#pragma system_header and bootstrap

[Phil Edwards]

> Think of it as a pair of bugs:  1) configure is (?) finding the wrong
> compiler (I haven't tested this myself), and 2) configure should be
> testing whatever compiler it finds to see if system_header is implemented,
> and disable -Werror if it isn't.  Both should be fixed; either would
> solve your problem.
>
> I'm working on the second one now...

Done, and in.

Phil