*: check defined(_WIN32) instead of defined(WIN32)

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

src/client/ClientFile.cxx

@@ -28,7 +28,7 @@
 void
 Client::AllowFile(Path path_fs) const
 {
-#ifdef WIN32
+#ifdef _WIN32
 	(void)path_fs;
 
 	throw ProtocolError(ACK_ERROR_PERMISSION, "Access denied");

src/client/ClientMessage.hxx

@@ -24,7 +24,7 @@
 
 #include <string>
 
-#ifdef WIN32
+#ifdef _WIN32
 /* fuck WIN32! */
 #include <windows.h>
 #undef GetMessage

src/client/ClientNew.cxx

@@ -29,7 +29,7 @@
 #include "Log.hxx"
 
 #include <assert.h>
-#ifdef WIN32
+#ifdef _WIN32
 #include <winsock2.h>
 #else
 #include <sys/socket.h>