__STRICT_ANSI__ "fixes" on STDC_0_IN_SYSTEM_HEADERS (solaris) targets

[Zack Weinberg]

> The runtime of what, gcc or fixincludes?  Whatever solution we come up
> with, I'd like to avoid duplicating setting STDC_0_IN_SYSTEM_HEADERS,
> i.e. bad idea to do it once in gcc and once in fixincludes.  Better is
> if we can include the target config file somehow.

Let's not go down the road of including the target config file in more
places which are not part of the compiler proper - which are not even
inside the gcc directory!

I'd like to see this used as an occasion to determine whether we still
need this particular hack, which I have never been fond of.  If we
really still do, the Right Thing would be to add another undocumented
built-in preprocessor macro, __STDC_0_IN_SYSTEM_HEADERS__ say, which
is defined only if CPP_OPTION(pfile, stdc_0_in_system_headers) is
true.

zw