libcurl 编译指南（支持SSL）

      首先，从libcurl官网下载源码：https://curl.haxx.se/download.html，我下载的是curl-7.48.0版本。其次，确认已经安装了VS2008或者其他版本的Visual Studio。

        由于之前使用VS编译的libcurl库不支持SSL，这次改用nmake来编译。经过测试，nmake编译出来的库是支持SSL的。至于VS编译出来的为什么不行（配置了预处理也不行），暂时无从知晓。下面就介绍如何使用nmake来编译出支持SSL的静态库：

        打开 Visual Studio 2008命令提示，cd 到 curl-7.48.0下的winbuild目录，然后按照winbuild目录下BUILD.WINDOWS.txt文件的说明，输入：nmake /f makefile.vc mode=static VC=9 ENABLE_IDN=no debug=no，然后回车开始编译！值得注意一下：最好指定VC的版本号（VS2008的VC版本是9.0），否则外部程序编译时，可能会出现“无法解析的外部符号”的提示错误。待编译完成后，在 curl-7.48.0\builds\libcurl-vc7-x86-release-static-ipv6-sspi-winssl 目录下，就是刚才编译好的lib库了。

======================== 编译选项 ===========================

Then you can call nmake /f Makefile.vc with the desired options (see below).  The builds will be in the top src directory, builds\ directory, in   a directory named using the options given to the nmake call.    nmake /f Makefile.vc mode=<static or dll> <options>    where <options> is one or many of:    VC=<6,7,8,9,10,11,12,14>     - VC versions    WITH_DEVEL=<path>            - Paths for the development files (SSL, zlib, etc.)                                   Defaults to sibbling directory deps: ../deps                                   Libraries can be fetched at http://windows.php.net/downloads/php-sdk/deps/                                   Uncompress them into the deps folder.    WITH_SSL=<dll or static>     - Enable OpenSSL support, DLL or static    WITH_CARES=<dll or static>   - Enable c-ares support, DLL or static    WITH_ZLIB=<dll or static>    - Enable zlib support, DLL or static    WITH_SSH2=<dll or static>    - Enable libSSH2 support, DLL or static    ENABLE_SSPI=<yes or no>      - Enable SSPI support, defaults to yes    ENABLE_IPV6=<yes or no>      - Enable IPv6, defaults to yes    ENABLE_IDN=<yes or no>       - Enable use of Windows IDN APIs, defaults to yes                                   Requires Windows Vista or later, or installation from:                                   https://www.microsoft.com/downloads/details.aspx?FamilyID=AD6158D7-DDBA-416A-9109-07607425A815    ENABLE_WINSSL=<yes or no>    - Enable native Windows SSL support, defaults to yes    GEN_PDB=<yes or no>          - Generate Program Database (debug symbols for release build)    DEBUG=<yes or no>            - Debug builds    MACHINE=<x86 or x64>         - Target architecture (default is x86)    Static linking of Microsoft's C RunTime (CRT):  ==============================================  If you are using mode=static nmake will create and link to the static build of  libcurl but *not* the static CRT. If you must you can force nmake to link in  the static CRT by passing RTLIBCFG=static. Typically you shouldn't use that  option, and nmake will default to the DLL CRT. RTLIBCFG is rarely used and  therefore rarely tested. When passing RTLIBCFG for a configuration that was  already built but not with that option, or if the option was specified  differently, you must destroy the build directory containing the configuration  so that nmake can build it from scratch.    Legacy Windows and SSL  ======================  When you build curl using the build files in this directory the default SSL  backend will be WinSSL (Windows SSPI, more specifically Schannel), the native  SSL library that comes with the Windows OS. WinSSL in Windows <= XP is not able  to connect to servers that no longer support the legacy handshakes and  algorithms used by those versions. If you will be using curl in one of those  earlier versions of Windows you should choose another SSL backend like OpenSSL.