diff --git a/m4/spice-compile-warnings.m4 b/m4/spice-compile-warnings.m4
index 1ecae2a..3c7e0d3 100644
--- a/m4/spice-compile-warnings.m4
+++ b/m4/spice-compile-warnings.m4
@@ -104,8 +104,12 @@ AC_DEFUN([SPICE_COMPILE_WARNINGS],[
     gl_WARN_ADD([-Wframe-larger-than=9216])
 
     # Use improved glibc headers
-    AC_DEFINE([_FORTIFY_SOURCE], [2],
-      [enable compile-time and run-time bounds-checking, and some warnings])
+    AH_VERBATIM([FORTIFY_SOURCE],
+    [/* Enable compile-time and run-time bounds-checking, and some warnings. */
+     #if defined __OPTIMIZE__ && __OPTIMIZE__
+     # define _FORTIFY_SOURCE 2
+     #endif
+    ])
 
     # Extra special flags
     dnl -fstack-protector stuff passes gl_WARN_ADD with gcc