Fix fenv.h fallback returns

[Ulrich Drepper]

On Thu, Jan 6, 2011 at 20:55, Joseph S. Myers <joseph@codesourcery.com> wrote:
> C99(+TC1) explicitly says that feclearexcept, feraiseexcept and
> fesetexceptflag return zero when their "excepts" operand is zero,
> because nothing needs to be done in that case.

This doesn't mean in the least that the implementation is wrong.
Additional error conditions can be defined and not supporting
exceptions at all definitely qualifies for that.  It's stupid to not
return an error.  People might get the impression that there is real
support.