*: 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/TimePrint.cxx

@@ -24,7 +24,7 @@
 void
 time_print(Response &r, const char *name, time_t t)
 {
-#ifdef WIN32
+#ifdef _WIN32
 	const struct tm *tm2 = gmtime(&t);
 #else
 	struct tm tm;
@@ -35,7 +35,7 @@ time_print(Response &r, const char *name, time_t t)
 
 	char buffer[32];
 	strftime(buffer, sizeof(buffer),
-#ifdef WIN32
+#ifdef _WIN32
 		 "%Y-%m-%dT%H:%M:%SZ",
 #else
 		 "%FT%TZ",