Disable _FORTIFY_SOURCE when enable-debug

Since enable-debug disables optimization entirely, _FORTIFY_SOURCE
does nothing as it requires some kind of optimization enabled. It
instead produces a bunch of useless warnings. So explicitly disable
it when enable-debug so that those warnings are not produced.

configure.ac

@@ -749,12 +749,17 @@ if test x$use_hardening != xno; then
   AX_CHECK_COMPILE_FLAG([-Wstack-protector],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -Wstack-protector"])
   AX_CHECK_COMPILE_FLAG([-fstack-protector-all],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-protector-all"])
 
-  AX_CHECK_PREPROC_FLAG([-D_FORTIFY_SOURCE=2],[
-    AX_CHECK_PREPROC_FLAG([-U_FORTIFY_SOURCE],[
-      HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -U_FORTIFY_SOURCE"
+  # When enable_debug is yes, all optimizations are disabled.
+  # However, FORTIFY_SOURCE requires that there is some level of optimization, otherwise it does nothing and just creates a compiler warning.
+  # Since FORTIFY_SOURCE is a no-op without optimizations, do not enable it when enable_debug is yes.
+  if test x$enable_debug != xyes; then
+    AX_CHECK_PREPROC_FLAG([-D_FORTIFY_SOURCE=2],[
+      AX_CHECK_PREPROC_FLAG([-U_FORTIFY_SOURCE],[
+        HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -U_FORTIFY_SOURCE"
+      ])
+      HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -D_FORTIFY_SOURCE=2"
     ])
-    HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -D_FORTIFY_SOURCE=2"
-  ])
+  fi
 
   AX_CHECK_LINK_FLAG([[-Wl,--dynamicbase]], [HARDENED_LDFLAGS="$HARDENED_LDFLAGS -Wl,--dynamicbase"])
   AX_CHECK_LINK_FLAG([[-Wl,--nxcompat]], [HARDENED_LDFLAGS="$HARDENED_LDFLAGS -Wl,--nxcompat"])