tool: Generate easysrc with last cache linked-list

Using a last cache linked-list improves the performance of easysrc
generation.

Bug: https://github.com/bagder/curl/issues/444
Ref: https://github.com/bagder/curl/issues/429

Closes #452

src/slist_wc.c

@@ -0,0 +1,73 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 1998 - 2015, Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at http://curl.haxx.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ***************************************************************************/
+
+#include "tool_setup.h"
+
+#ifndef CURL_DISABLE_LIBCURL_OPTION
+
+#include "slist_wc.h"
+
+/* The last #include files should be: */
+#include "curl_memory.h"
+#include "memdebug.h"
+
+/*
+ * slist_wc_append() appends a string to the linked list. This function can be
+ * used as an initialization function as well as an append function.
+ */
+struct slist_wc *slist_wc_append(struct slist_wc *list,
+                                 const char *data)
+{
+  struct curl_slist *new_item = curl_slist_append(NULL, data);
+
+  if(!new_item)
+    return NULL;
+
+  if(!list) {
+    list = malloc(sizeof(struct slist_wc));
+
+    if(!list) {
+      free(new_item);
+      return NULL;
+    }
+
+    list->first = new_item;
+    list->last = new_item;
+    return list;
+  }
+
+  list->last->next = new_item;
+  list->last = list->last->next;
+  return list;
+}
+
+/* be nice and clean up resources */
+void slist_wc_free_all(struct slist_wc *list)
+{
+  if(!list)
+    return;
+
+  curl_slist_free_all(list->first);
+  free(list);
+}
+
+#endif /* CURL_DISABLE_LIBCURL_OPTION */