[v3] C headers, part 1

[Loren James Rittle]

> This is a slightly modified version of this patch:
>
> http://gcc.gnu.org/ml/gcc-patches/2001-05/msg01349.html

Yow!  I am going to have to completely rework the configuration part
of the thread patch in light of this massive patch.  I will need help
since this patch changes all the rules, I just had to learn the hard way.

For example, it appears that under this new set of rules that
AC_LINK_FILE is basically disallowed to stage header files.

Regards,
Loren