Replace WIN32 with _WIN32

Using WIN32 depends on the build environment defining the variable.
_WIN32 is a predefined MSVC macro and is always available.

Signed-off-by: Tim Crawford <crawfxrd@gmail.com>

lib/socket.c

@@ -43,7 +43,7 @@
 #include <sys/socket.h>
 #endif
 
-#if defined(WIN32)
+#if defined(_WIN32)
 #include <winsock2.h>
 #include <ws2tcpip.h>
 #include "win32/win32_compat.h"
@@ -139,7 +139,7 @@ void iscsi_decrement_iface_rr() {
 
 static int set_nonblocking(int fd)
 {
-#if defined(WIN32)
+#if defined(_WIN32)
 	unsigned long opt = 1;
 	return ioctlsocket(fd, FIONBIO, &opt);
 #else
@@ -272,7 +272,7 @@ static int iscsi_tcp_connect(struct iscsi_context *iscsi, union socket_address *
 	}
 
 	if (connect(iscsi->fd, &sa->sa, socksize) != 0
-#if defined(WIN32)
+#if defined(_WIN32)
             && WSAGetLastError() != WSAEWOULDBLOCK
 #endif
             && errno != EINPROGRESS) {