Fix compiler warnings on Windows due to redefines (#258)

Co-authored-by: Arvid Norlander <[email protected]>

include/enet/win32.h

@@ -11,10 +11,14 @@
 #pragma warning (disable: 4244) // 64bit to 32bit int
 #pragma warning (disable: 4018) // signed/unsigned mismatch
 #pragma warning (disable: 4146) // unary minus operator applied to unsigned type
+#ifndef _CRT_SECURE_NO_DEPRECATE
 #define _CRT_SECURE_NO_DEPRECATE
+#endif
+#ifndef _CRT_SECURE_NO_WARNINGS
 #define _CRT_SECURE_NO_WARNINGS
 #endif
 #endif
+#endif
 
 #include <stdlib.h>
 #include <winsock2.h>