Re-enable warnings for libcpp (build maintainer review requested)

[Nathanael Nerode]

DJ Delorie wrote:
>>I don't think we have a naming convention for gcc+src-wide autoconf
>>macros.
> 
> 
> No, I don't think we do either, and I don't have a better suggestion :-P
OK, this is going to sound stupid, but how about

TL_*

It's short and stands for "Top level", which is what everyone calls it.
Nobody else is using it as far as I can tell.

Meanwhile, I'm trying to use

ACX_*

for things which we think "oughtta be in autoconf".

If both choices sound good, I'll document them somewhere... likely the 
most logical place is actually in a config/README or some such.