diff --git a/autogen/config.in b/autogen/config.in
index c5f32371dd7..8fe2170d89d 100644
--- a/autogen/config.in
+++ b/autogen/config.in
@@ -144,7 +144,7 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 
 /* Enable compile-time and run-time bounds-checking, and some warnings,
       without upsetting glibc 2.15+. */
-   #if defined __OPTIMIZE__ && __OPTIMIZE__
+   #if !defined _FORTIFY_SOURCE && defined __OPTIMIZE__ && __OPTIMIZE__
    # define _FORTIFY_SOURCE 2
    #endif