configure: check if -latomic is needed for __atomic_*

On some platforms, gcc generates library calls when __atomic_* functions
are used, but does not link the required library (libatomic) automatically
(supposedly to allow the app to use some other atomics implementation?).

Detect this at configure time and add the library when needed. Tested
on armel (library was added) and on x86_64 (was not, as expected).

Some documentation on this is provided in GCC wiki:
https://gcc.gnu.org/wiki/Atomic/GCCMM

Fixes: 8915f0c0 "util: use GCC atomic intrinsics with explicit memory model"
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102573
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 2ef7f23820)

configure.ac

@@ -410,8 +410,21 @@ int main() {
 }]])], GCC_ATOMIC_BUILTINS_SUPPORTED=1)
 if test "x$GCC_ATOMIC_BUILTINS_SUPPORTED" = x1; then
     DEFINES="$DEFINES -DUSE_GCC_ATOMIC_BUILTINS"
+    dnl On some platforms, new-style atomics need a helper library
+    AC_MSG_CHECKING(whether -latomic is needed)
+    AC_LINK_IFELSE([AC_LANG_SOURCE([[
+    #include <stdint.h>
+    uint64_t v;
+    int main() {
+        return (int)__atomic_load_n(&v, __ATOMIC_ACQUIRE);
+    }]])], GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC=no, GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC=yes)
+    AC_MSG_RESULT($GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC)
+    if test "x$GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC" = xyes; then
+        LIBATOMIC_LIBS="-latomic"
+    fi
 fi
 AM_CONDITIONAL([GCC_ATOMIC_BUILTINS_SUPPORTED], [test x$GCC_ATOMIC_BUILTINS_SUPPORTED = x1])
+AC_SUBST([LIBATOMIC_LIBS])
 
 dnl Check if host supports 64-bit atomics
 dnl note that lack of support usually results in link (not compile) error

src/util/Makefile.am

@@ -46,7 +46,8 @@ libmesautil_la_SOURCES = \
 
 libmesautil_la_LIBADD = \
 	$(CLOCK_LIB) \
-	$(ZLIB_LIBS)
+	$(ZLIB_LIBS) \
+	$(LIBATOMIC_LIBS)
 
 u_atomic_test_LDADD = libmesautil.la
 roundeven_test_LDADD = -lm