bug#42860: 28.0.50; Build on OpenBSD fails in jansson.h

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Fri, 14 Aug 2020 16:29:41 +0200
> Cc: Philipp Stephani <phst@google.com>
> 
> 
> This is on OpenBSD 6.6, with "pkg_add jansson", build from Emacs trunk now:
> 
> gmake[1]: Entering directory '/home/larsi/src/emacs/trunk/src'
>   CC       json.o
> In file included from json.c:27:
> /usr/local/include/jansson.h: In function 'json_incref':
> /usr/local/include/jansson.h:124: warning: implicit declaration of function '__atomic_add_fetch'
> /usr/local/include/jansson.h:124: error: '__ATOMIC_ACQUIRE' undeclared (first use in this function)
> /usr/local/include/jansson.h:124: error: (Each undeclared identifier is reported only once
> /usr/local/include/jansson.h:124: error: for each function it appears in.)
> /usr/local/include/jansson.h: In function 'json_decref':
> /usr/local/include/jansson.h:134: warning: implicit declaration of function '__atomic_sub_fetch'
> /usr/local/include/jansson.h:134: error: '__ATOMIC_RELEASE' undeclared (first use in this function)
> gmake[1]: *** [Makefile:401: json.o] Error 1
> gmake[1]: Leaving directory '/home/larsi/src/emacs/trunk/src'
> gmake: *** [Makefile:424: src] Error 2
> *** Error 2 in /home/larsi/src/emacs/trunk (makefile:4 'all')
> 
> I haven't investigated further -- I wonder whether it's immediately
> obvious to Philipp what the problem is here.  :-)

Too old GCC version?  What do you have there?