threads-h: Fix compilation errors in C++ mode on Android

[Bruno Haible]

In a testdir on Android, I see compilation errors like this one:

clang++ -ferror-limit=0 -DHAVE_CONFIG_H -DEXEEXT=\"\" -DEXEEXT=\"\" -I. 
-I../../gltests -I..  -DGNULIB_STRICT_CHECKING=1 -DIN_GNULIB_TESTS=1 -I. 
-I../../gltests -I.. -I../../gltests/.. -I../gllib -I../../gltests/../gllib 
-I/data/data/com.termux/files/home/local/include -Wall  -Wno-error -g -O2 -c -o 
test-threads-c++.o ../../gltests/test-threads-c++.cc

In file included from ../../gltests/test-threads-c++.cc:22:
In file included from ../gllib/threads.h:28:
In file included from /data/data/com.termux/files/usr/include/threads.h:229:
In file included from 
/data/data/com.termux/files/usr/include/android/legacy_threads_inlines.h:36:
In file included from 
/data/data/com.termux/files/usr/include/bits/threads_inlines.h:31:
../gllib/threads.h:699:19: error: no member named 'thrd_create' in the global 
namespace; did you mean 'pthread_create'?
_GL_CXXALIAS_SYS (thrd_create, int, (thrd_t *, thrd_start_t, void *));
~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The first patch fixes them. The second patch is merely a portability
improvement.


2023-01-09  Bruno Haible  <bruno@clisp.org>

        threads-h: Improve portability.
        * lib/threads.in.h: Use INCLUDE_NEXT_AS_FIRST_DIRECTIVE when possible.
        * modules/threads-h (Makefile.am): Substitute
        INCLUDE_NEXT_AS_FIRST_DIRECTIVE and NEXT_AS_FIRST_DIRECTIVE_THREADS_H.

2023-01-09  Bruno Haible  <bruno@clisp.org>

        threads-h: Fix compilation errors in C++ mode on Android.
        * lib/threads.in.h (_GL_ALREADY_INCLUDING_THREADS_H): New macro.

0001-threads-h-Fix-compilation-errors-in-C-mode-on-Androi.patch
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0002-threads-h-Improve-portability.patch
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