configure.ac: define WINVER

Ensure that WINVER is defined early enough, so other system headers
won't fall back to their default value.  Specifically, this solves a
build failure (-Werror) with mingw-w64 ("WINVER redefined").

src/main_win32.c

@@ -27,7 +27,6 @@
 
 #include <glib.h>
 
-#define WINVER 0x0501
 #include <windows.h>
 
 static int service_argc;

src/server_socket.c

@@ -34,7 +34,6 @@
 #include <assert.h>
 
 #ifdef WIN32
-#define WINVER 0x0501
 #include <ws2tcpip.h>
 #include <winsock.h>
 #else

src/socket_util.c

@@ -28,7 +28,6 @@
 #include <sys/socket.h>
 #include <netdb.h>
 #else /* G_OS_WIN32 */
-#define WINVER 0x0501
 #include <ws2tcpip.h>
 #include <winsock.h>
 #endif /* G_OS_WIN32 */