check for _WIN32 instead of WIN32

include/enet/enet.h

@@ -12,7 +12,7 @@ extern "C"
 
 #include <stdlib.h>
 
-#ifdef WIN32
+#ifdef _WIN32
 #include "enet/win32.h"
 #else
 #include "enet/unix.h"

win32.c

@@ -2,7 +2,7 @@
  @file  win32.c
  @brief ENet Win32 system specific functions
 */
-#ifdef WIN32
+#ifdef _WIN32
 
 #include <time.h>
 #define ENET_BUILDING_LIB 1