diff --git a/configure.ac b/configure.ac
index e5e05be5c..6504144ee 100644
--- a/configure.ac
+++ b/configure.ac
@@ -170,7 +170,7 @@ if test "$gl_gcc_warnings" = yes; then
   AH_VERBATIM([FORTIFY_SOURCE],
   [/* 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
   ])