Remove __STDC__ conditionals from installed headers

[Roland McGrath]

I only skimmed the changes so I won't have noticed any typos or suchlike.
That said, it looks fine to me.  These changes should have zero effect on
the code generated, so e.g. a stripped libm.so should be identical before
and after (modulo build ID).  So perhaps you could verify that's so.
But assuming no unnoticed mistakes, I'd say put it in.


Thanks,
Roland