*: 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/LogInit.cxx

@@ -72,7 +72,7 @@ log_init_file(int line)
 
 	out_fd = open_log_file();
 	if (out_fd < 0) {
-#ifdef WIN32
+#ifdef _WIN32
 		const std::string out_path_utf8 = out_path.ToUTF8();
 		throw FormatRuntimeError("failed to open log file \"%s\" (config line %d)",
 					 out_path_utf8.c_str(), line);
@@ -182,7 +182,7 @@ void setup_log_output()
 	fflush(nullptr);
 
 	if (out_fd < 0) {
-#ifdef WIN32
+#ifdef _WIN32
 		return;
 #else
 		out_fd = open("/dev/null", O_WRONLY);