threads-h: Don't override the C++ keyword 'thread_local'

[Bruno Haible]

clang on native Windows produces the warning:

In file included from 
C:\cygwin64\home\bruno\testdir-all-for-msvc\gltests\test-threads-c++.cc:22:
../gllib\threads.h(685,10): warning: keyword is hidden by macro definition 
[-Wkeyword-macro]

The cause is that in C++ 11 or newer, 'thread_local' is a keyword, and
there is no need to redefine it as a macro.

This patch fixes it.


2024-08-28  Bruno Haible  <bruno@clisp.org>

        threads-h: Don't override the C++ keyword 'thread_local'.
        * lib/threads.in.h (thread_local): Don't define as a macro in C++ 11
        or newer.

diff --git a/lib/threads.in.h b/lib/threads.in.h
index 4e21fc7628..6a52e7b795 100644
--- a/lib/threads.in.h
+++ b/lib/threads.in.h
@@ -97,7 +97,9 @@
    its expansion ends in a semicolon.  */
 # undef thread_local
 #endif
-#if !@HAVE_THREADS_H@ || !defined thread_local
+#if (!@HAVE_THREADS_H@ || !defined thread_local) \
+    && !(defined __cplusplus \
+         && (__cplusplus >= 201103L || (defined _MSC_VER && _MSC_VER >= 1900)))
 # define thread_local _Thread_local
 #endif
 /* Define the macro thread_local if and only if it actually works.  */