threads-h: Define 'thread_local' if and only it actually works.

* m4/threads.m4 (gl_THREAD_LOCAL_DEFINITION): New macro.
(gl_THREADS_H): Define _Thread_local to __thread also for ARM C, IBM C,
Oracle Solaris Studio C. Compile a simple program, to see whether
_Thread_local basically works. Set HAVE_THREAD_LOCAL and LIBTHREADLOCAL.
(gl_THREADS_H_DEFAULTS): Initialize HAVE_THREAD_LOCAL.
* lib/threads.in.h (thread_local): Undefine if it does not work.
* modules/threads-h (Makefile.am): Substitute HAVE_THREAD_LOCAL.
(Link): Mention LIBTHREADLOCAL.
* tests/test-threads.c: Don't check that thread_local is defined.
* tests/test-thread_local.c: New file.
* modules/threads-h-tests (Files): Add it and macros.h.
(Depends-on): Add thrd and stdint.
(configure.ac): Test whether 'alarm' is declared.
(Makefile.am): Arrange to build and link test-thread_local.
* doc/posix-headers/threads.texi: Mention the platforms that don't
support 'thread_local'.

1 files changed, 14 insertions, 0 deletions

diff --git a/doc/posix-headers/threads.texi b/doc/posix-headers/threads.texi
index 71ae43a48b..3c1c9fdc62 100644
--- a/doc/posix-headers/threads.texi
+++ b/doc/posix-headers/threads.texi
@@ -20,4 +20,18 @@ AIX 7.2.
 
 Portability problems not fixed by Gnulib:
 @itemize
+@item
+There is no way to define a working @code{thread_local} macro on some platforms:
+@itemize
+@item
+Mac OS X 10.5,
+@item
+OpenBSD 6.5,
+@item
+AIX 7.1 with gcc (but it works with @samp{xlc -qthreaded -qtls}),
+@item
+HP-UX 11.31 with cc (but it works with gcc),
+@item
+IRIX 6.5.
+@end itemize
 @end itemize