diff --git a/m4/ax_atomic.m4 b/m4/ax_atomic.m4
index 7d6dae261..b1fa58753 100644
--- a/m4/ax_atomic.m4
+++ b/m4/ax_atomic.m4
@@ -31,6 +31,6 @@ AC_DEFUN([AC_C___ATOMIC],
 if test $ac_cv_c___atomic = yes; then
   AC_DEFINE([HAVE_C___ATOMIC], 1,
            [Define to 1 if __atomic operations work.])
-  AM_CFLAGS="$AM_CFLAGS -DHAVE_C___ATOMIC"
+  AM_CFLAGS="$AM_CFLAGS -DHAVE_C___ATOMIC=1"
 fi
 ])# AC_C___ATOMIC