fix dependencies among thread modules

[Bruno Haible]

The general structure of the multithreading modules in gnulib is that we have
three set of modules:

  pthread-*, sched_yield : POSIX API

  threads, threads-h, thrd, mtx, cnd, tss: ISO C API

  threadlib, thread, lock, cond, tls, yield:
    gnulib API, obeys --enable-threads configure option

They are independent, except for the fact that the *implementation* of the
second and third set may use the POSIX API.

There are currently two violations of this structure:

1) The threads-h modules should not depend on threadlib.

2) sched_yield should not depend on threadlib and yield.
   It's the other way around: yield may depend on sched_yield.

These patches fixes them.


2019-12-21  Bruno Haible  <address@hidden>

        threads-h: Don't depend on threadlib.
        * modules/threads-h (configure.ac-early): Invoke gl_ANYTHREADLIB_EARLY,
        not gl_THREADLIB_EARLY.

2019-12-21  Bruno Haible  <address@hidden>

        sched_yield: Don't depend on threadlib and yield.
        * m4/threadlib.m4 (gl_PTHREADLIB): Document that it sets
        LIB_SCHED_YIELD.
        (gl_PTHREADLIB_BODY): Set LIB_SCHED_YIELD (code moved here from
        m4/yield.m4).
        * m4/sched_yield.m4 (gl_FUNC_SCHED_YIELD): Require gl_PTHREADLIB, not
        gl_THREADLIB and gl_YIELD.
        * m4/yield.m4 (gl_YIELD): Require gl_PTHREADLIB. Determine YIELD_LIB
        based on $(LIB_SCHED_YIELD).
        * m4/threads.m4 (gl_THREADS_H): Don't require gl_YIELD. Use
        $(LIB_SCHED_YIELD), not $(YIELD_LIB).
        * modules/sched_yield (Files): Remove yield.m4. Add threadlib.m4.
        (Depends-on): Remove threadlib.
        (Link): Mention $(LIB_SCHED_YIELD), not $(YIELD_LIB).
        * modules/threads-h (Files): Remove m4/yield.m4.
        * modules/pthread-cond-tests (Makefile.am): Link the programs against
        $(LIB_SCHED_YIELD), not $(YIELD_LIB).
        * modules/pthread-mutex-tests (Makefile.am): Likewise.
        * modules/pthread-once-tests (Makefile.am): Likewise.
        * modules/pthread-rwlock-tests (Makefile.am): Likewise.
        * modules/pthread-tss-tests (Makefile.am): Likewise.

0001-threads-h-Don-t-depend-on-threadlib.patch
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0002-sched_yield-Don-t-depend-on-threadlib-and-yield.patch
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