slow V3 configures (was Re: ICE during bootstrap.)

[Benjamin Kosnik]

> We're pretty certain that those all need to be there, I think.  Whenever we
> start making assumption about "if foo() is there, surely bar() is also
> there," it breaks on somebody's platform.

Right. It's a pain, but necessary. We try to cache most of it, but there 
are places where it could be speeded up.

>     I submit that those are the exceptions rather than the rule.  For the
>     math support, we already just tweak the appropriate cxxflags.  If there
>     are tests that /must/ be run as C rather than C++, then we can enclose
>     /those/ in LANG_SAVE, LANG_C, LANG_RESTORE.

we check for c++ declarations and c linkage. both are necessary. If you 
can figure out a faster way to do it, I'm all ears.

-benjamin