* lib/sched.in.h: Add _GL_FUNCDECL_RPL, _GL_WARN_ON_USE placeholders.
(sched_yield): New declaration.
* lib/sched_yield.c: New file.
* m4/sched_yield.m4: New file.
* m4/sched_h.m4 (gl_SCHED_H): Require gl_SCHED_H_DEFAULTS. Arrange to
provide a replacement sched.h always. Test whether sched_yield is
declared.
(gl_SCHED_MODULE_INDICATOR, gl_SCHED_H_DEFAULTS): New macros.
* modules/sched (Depends-on): Add snippet/c++defs, snippet/warn-on-use.
(Makefile.am): Provide a replacement sched.h always. Substitute
GNULIB_SCHED_YIELD, HAVE_SCHED_YIELD, REPLACE_SCHED_YIELD,
_GL_FUNCDECL_RPL, _GL_WARN_ON_USE.
* modules/sched_yield: New file.
* doc/posix-functions/sched_yield.texi: Mention the new module.