windows: simplify detecting and using system headers

- autotools, cmake: assume that if we detect Windows, `windows.h`,
  `winsock2.h` and `ws2tcpip.h` do exist.
- lib: fix 3 outlier `#if` conditions to use `USE_WINSOCK` instead of
  looking for `winsock2.h`.
- autotools: merge 3 Windows check methods into one.
- move Watt-32 and lwIP socket support to `setup-win32.h` from
  `config-win32.h`. It opens up using these with all build tools. Also
  merge logic with Windows Sockets.
- fix to assume Windows sockets with the mingw32ce toolchain.
  Follow-up to: 2748c64d60
- cmake: delete unused variable `signature_call_conv` since
  eb33ccd533.
- autotools: simplify `CURL_CHECK_WIN32_LARGEFILE` detection.
- examples/externalsocket: fix header order.
- cmake/OtherTests.cmake: delete Windows-specific `_source_epilogue`
  that wasn't used anymore.
- cmake/OtherTests.cmake: set `WIN32_LEAN_AND_MEAN` for test
  `SIZEOF_STRUCT_SOCKADDR_STORAGE`.

After this patch curl universally uses `_WIN32` to guard
Windows-specific logic. It guards Windows Sockets-specific logic with
`USE_WINSOCK` (this might need further work).

Reviewed-by: Jay Satiro
Closes #12495

lib/cf-socket.c

@@ -137,14 +137,14 @@ static void nosigpipe(struct Curl_easy *data,
 #define nosigpipe(x,y) Curl_nop_stmt
 #endif
 
-#if defined(__DragonFly__) || defined(HAVE_WINSOCK2_H)
+#if defined(__DragonFly__) || defined(USE_WINSOCK)
 /* DragonFlyBSD and Windows use millisecond units */
 #define KEEPALIVE_FACTOR(x) (x *= 1000)
 #else
 #define KEEPALIVE_FACTOR(x)
 #endif
 
-#if defined(HAVE_WINSOCK2_H) && !defined(SIO_KEEPALIVE_VALS)
+#if defined(USE_WINSOCK) && !defined(SIO_KEEPALIVE_VALS)
 #define SIO_KEEPALIVE_VALS    _WSAIOW(IOC_VENDOR,4)
 
 struct tcp_keepalive {