build: Undef _FORTIFY_SOURCE for unoptimized builds

'./configure --disable-optimization --enable-maintainer-mode' would
result in build errors. _FORTIFY_SOURCE requires that optimization is
enabled and issues a compile-time warning which is treated as an
error. Disabling optimization takes precedence over fortification in
this case, since it is being explicitly requested.

configure.ac

@@ -30,7 +30,7 @@ LT_INIT([disable-static])
 AC_ARG_ENABLE(optimization, AC_HELP_STRING([--disable-optimization],
 			[disable code optimization through compiler]), [
 	if (test "${enableval}" = "no"); then
-		CFLAGS="$CFLAGS -O0"
+		CFLAGS="$CFLAGS -U_FORTIFY_SOURCE -O0"
 	fi
 ])