fix compilation of core/bricks with clang++10

* configure.ac: Call CHECK_ATOMIC after setting the C++ options.
This is probably not necessary, but it makes more sense.
* m4/l_atomic.m4: Augment the test body with a scenario
that actually requires -latomic.

configure.ac

@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
-# Copyright (C) 2008-2020, Laboratoire de Recherche et Développement
+# Copyright (C) 2008-2021, Laboratoire de Recherche et Développement
 # de l'Epita (LRDE).
 # Copyright (C) 2003-2007 Laboratoire d'Informatique de Paris 6
 # (LIP6), département Systèmes Répartis Coopératifs (SRC), Université
@@ -79,9 +79,6 @@ AX_CHECK_COMPILE_FLAG([-std=c11], [CFLAGS="$CFLAGS -std=c11"])
 
 gl_INIT
 
-dnl Check if -latomic is required for <std::atomic>
-CHECK_ATOMIC
-
 # Use -Werror since using -fvisibility under MinGW is only a warning.
 # (The option is ignored anyway since this does not make sense under windows).
 AX_CHECK_COMPILE_FLAG([-Werror -fvisibility=hidden],
@@ -145,6 +142,9 @@ else
   fi
 fi
 
+dnl Check if -latomic is required for <std::atomic>
+CHECK_ATOMIC
+
 AX_CHECK_BUDDY
 
 AC_CHECK_HEADERS([sys/times.h valgrind/memcheck.h spawn.h])