加载外部库时遇到麻烦

我在c++程序中使用curl库时遇到了麻烦。我将curl文件夹直接复制到Microsoft Visual Studio 12.0VCcrtsrc文件夹中。我使用的测试程序直接从Curl的网站，所以不应该有任何问题。然而，它确实给了我一些未解决的外部符号，看起来像库没有正确加载。我正在使用visual studio 12.0 express.

`/***************************************************************************
*                                  _   _ ____  _
*  Project                     ___| | | |  _ | |
*                             / __| | | | |_) | |
*                            | (__| |_| |  _ <| |___
*                             ___|___/|_| ______|
*
* Copyright (C) 1998 - 2014, 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 <stdio.h>
#include <string.h>
#include "curlcurl.h"
/* This is a simple example showing how to send mail using libcurl's SMTP
* capabilities. It builds on the smtp-mail.c example to add authentication
* and, more importantly, transport security to protect the authentication
* details from being snooped.
*
* Note that this example requires libcurl 7.20.0 or above.
*/
#define FROM    "<sender@example.org>"
#define TO      "<addressee@example.net>"
#define CC      "<info@example.org>"
static const char *payload_text[] = {
    "Date: Mon, 29 Nov 2010 21:54:29 +1100rn",
    "To: " TO "rn",
    "From: " FROM "(Example User)rn",
    "Cc: " CC "(Another example User)rn",
    "Message-ID: <dcd7cb36-11db-487a-9f3a-e652a9458efd@rfcpedant.example.org>rn",
    "Subject: SMTP TLS example messagern",
    "rn", /* empty line to divide headers from body, see RFC5322 */
    "The body of the message starts here.rn",
    "rn",
    "It could be a lot of lines, could be MIME encoded, whatever.rn",
    "Check RFC5322.rn",
    NULL
};
struct upload_status {
    int lines_read;
};
static size_t payload_source(void *ptr, size_t size, size_t nmemb, void *userp)
{
    struct upload_status *upload_ctx = (struct upload_status *)userp;
    const char *data;
    if ((size == 0) || (nmemb == 0) || ((size*nmemb) < 1)) {
        return 0;
    }
    data = payload_text[upload_ctx->lines_read];
    if (data) {
        size_t len = strlen(data);
        memcpy(ptr, data, len);
        upload_ctx->lines_read++;
        return len;
    }
    return 0;
}
int main(void)
{
    CURL *curl;
    CURLcode res = CURLE_OK;
    struct curl_slist *recipients = NULL;
    struct upload_status upload_ctx;
    upload_ctx.lines_read = 0;
    curl = curl_easy_init();
    if (curl) {
        /* Set username and password */
        curl_easy_setopt(curl, CURLOPT_USERNAME, "user");
        curl_easy_setopt(curl, CURLOPT_PASSWORD, "secret");
        /* This is the URL for your mailserver. Note the use of port 587 here,
        * instead of the normal SMTP port (25). Port 587 is commonly used for
        * secure mail submission (see RFC4403), but you should use whatever
        * matches your server configuration. */
        curl_easy_setopt(curl, CURLOPT_URL, "smtp://mainserver.example.net:587");
        /* In this example, we'll start with a plain text connection, and upgrade
        * to Transport Layer Security (TLS) using the STARTTLS command. Be careful
        * of using CURLUSESSL_TRY here, because if TLS upgrade fails, the transfer
        * will continue anyway - see the security discussion in the libcurl
        * tutorial for more details. */
        curl_easy_setopt(curl, CURLOPT_USE_SSL, (long)CURLUSESSL_ALL);
        /* If your server doesn't have a valid certificate, then you can disable
        * part of the Transport Layer Security protection by setting the
        * CURLOPT_SSL_VERIFYPEER and CURLOPT_SSL_VERIFYHOST options to 0 (false).
        *   curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L);
        *   curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0L);
        * That is, in general, a bad idea. It is still better than sending your
        * authentication details in plain text though.
        * Instead, you should get the issuer certificate (or the host certificate
        * if the certificate is self-signed) and add it to the set of certificates
        * that are known to libcurl using CURLOPT_CAINFO and/or CURLOPT_CAPATH. See
        * docs/SSLCERTS for more information. */
        curl_easy_setopt(curl, CURLOPT_CAINFO, "/path/to/certificate.pem");
        /* Note that this option isn't strictly required, omitting it will result in
        * libcurl sending the MAIL FROM command with empty sender data. All
        * autoresponses should have an empty reverse-path, and should be directed
        * to the address in the reverse-path which triggered them. Otherwise, they
        * could cause an endless loop. See RFC 5321 Section 4.5.5 for more details.
        */
        curl_easy_setopt(curl, CURLOPT_MAIL_FROM, FROM);
        /* Add two recipients, in this particular case they correspond to the
        * To: and Cc: addressees in the header, but they could be any kind of
        * recipient. */
        recipients = curl_slist_append(recipients, TO);
        recipients = curl_slist_append(recipients, CC);
        curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);
        /* We're using a callback function to specify the payload (the headers and
        * body of the message). You could just use the CURLOPT_READDATA option to
        * specify a FILE pointer to read from. */
        curl_easy_setopt(curl, CURLOPT_READFUNCTION, payload_source);
        curl_easy_setopt(curl, CURLOPT_READDATA, &upload_ctx);
        curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);
        /* Since the traffic will be encrypted, it is very useful to turn on debug
        * information within libcurl to see what is happening during the transfer.
        */
        curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
        /* Send the message */
        res = curl_easy_perform(curl);
        /* Check for errors */
        if (res != CURLE_OK)
            fprintf(stderr, "curl_easy_perform() failed: %sn",
            curl_easy_strerror(res));
        /* Free the list of recipients */
        curl_slist_free_all(recipients);
        /* Always cleanup */
        curl_easy_cleanup(curl);
    }
    return (int)res;
}`