autotools: include win32 specific headers for socklen_t detection on win32/mingw

autotools: include win32 specific headers for socklen_t detection on win32/mingw

The [1] removes EVENT__ prefix, and now if we will incorrectly detect
that "foobar" (or socklen_t in our case) type is not available, but
somewhere later it will be available then we will get next error [2]:
  error: two or more data types in declaration specifiers

According to [3]:
- Compile something in Cygwin and you are compiling it for Cygwin.
- Compile something in MinGW and you are compiling it for Windows.
  And I can confirm this, since there is _WIN32 defined (according to [4])

And since according to [5] our image in appveyour (Visual Studion 2015)
has mingw (and we use it, not cygwin) we need ws2tcpip.h (over
sys/socket.h -- which does not exist in win32) header to detect
socklen_t existence.

[1]: 587e9f5828c4eb3c6e79e9eb29f174e4ae0c05bd ("config.h can't be prefixed unconditionally")
[2]: https://ci.appveyor.com/project/nmathewson/libevent/build/job/yonukoc5q3tr3e5e#L372
[3]: https://stackoverflow.com/questions/771756/what-is-the-difference-between-cygwin-and-mingw
[4]: https://www.appveyor.com/docs/build-environment/#mingw-msys-cygwin
[5]: https://ci.appveyor.com/project/nmathewson/libevent/build/job/yonukoc5q3tr3e5e#L164

Fixes: #649