change autoconf #include in configure.in

Updates "system.h" and "replacements.h" with <helpers/system.h> and
<helpers/replacements.h> respectively.

configure.in

@@ -8,8 +8,8 @@ AM_MAINTAINER_MODE
 
 AM_CONFIG_HEADER(config.h)
 AH_BOTTOM([
-#include "system.h"
-#include "replacements.h"
+#include <helper/system.h>
+#include <helper/replacements.h>
 ])
 
 AC_LANG_C