Problem reported by Denis Davydov in:
http://lists.gnu.org/archive/html/bug-gnulib/2016-09/msg00056.html
* lib/sched.in.h [HAVE_SYS_CDEFS_H]:
Include <sys/cdefs.h> before <sched.h>.
* m4/nproc.m4 (gl_PREREQ_NPROC): Include errno.h before sched.h,
so that we needn’t worry about the sched.h include bug here.
* m4/sched_h.m4 (gl_SCHED_H): Check for sys/cdefs.h,
and include it before <sched.h> if it exists, when
checking for <sched.h>.