fix windows build under mingw32
fix windows build under mingw32

`_WIN32_WINNT` is not set under mingw32. set its value to match Windows XP.

losingle/seafile-client,lucius-feng/seafile-client,wgaalves/seafile-client,losingle/seafile-client,daodaoliang/seafile-client,lucius-feng/seafile-client,lucius-feng/seafile-client,haiwen/seafile-client,daodaoliang/seafile-client,haiwen/seafile-client,haiwen/seafile-client,daodaoliang/seafile-client,wgaalves/seafile-client,wgaalves/seafile-client,losingle/seafile-client,daodaoliang/seafile-client,wgaalves/seafile-client,losingle/seafile-client,lucius-feng/seafile-client,haiwen/seafile-client