Don't use the identifier 'noreturn'.

Fixes <http://bugs.gnu.org/15798>.
Reported by Matt Sicker <boards@gmail.com>.

* libguile/__scm.h (SCM_NORETURN): Use ((__noreturn__)) instead of
  ((noreturn)).

* libguile/throw.h (scm_ithrow): Rename formal parameter from 'noreturn'
  to 'no_return'.

* libguile/throw.c (scm_ithrow): Rename formal parameter from 'noreturn'
  to 'no_return'.