GCC and system headers

[Martin v. Loewis]

> could somebody explain what are the technical reasons for
> not allowing ports to choose if they want to use standard
> headers provided by GCC? Many times I've seen on this
> mailing list that 'overriding USER_H is a brain-damaged 
> feature' without explanation *how* it is brain-damaged.

Please elaborate; I'm not aware of any prior discussion of this topic.
What exactly is user.h, and what does it have to do with ports? And
who said that they must not choose whether they want to use standard
headers?

Regards,
Martin