diff --git a/acinclude.m4 b/acinclude.m4
index 0c308bc..625413f 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -46,7 +46,7 @@ AC_DEFUN([VIRT_VIEWER_COMPILE_WARNINGS],[
 
     AH_VERBATIM([FORTIFY_SOURCE],
                 [/* Enable compile-time and run-time bounds-checking, and some warnings. */
-                 #if defined __OPTIMIZE__ && __OPTIMIZE__
+                 #if !defined _FORTIFY_SOURCE && defined __OPTIMIZE__ && __OPTIMIZE__
                  # define _FORTIFY_SOURCE 2
                  #endif
                 ])