*: 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/fs/FileSystem.cxx

@@ -29,7 +29,7 @@
 void
 RenameFile(Path oldpath, Path newpath)
 {
-#ifdef WIN32
+#ifdef _WIN32
 	if (!MoveFileEx(oldpath.c_str(), newpath.c_str(),
 			MOVEFILE_REPLACE_EXISTING))
 		throw MakeLastError("Failed to rename file");
@@ -42,7 +42,7 @@ RenameFile(Path oldpath, Path newpath)
 AllocatedPath
 ReadLink(Path path)
 {
-#ifdef WIN32
+#ifdef _WIN32
 	(void)path;
 	errno = EINVAL;
 	return AllocatedPath::Null();
@@ -63,7 +63,7 @@ ReadLink(Path path)
 void
 TruncateFile(Path path)
 {
-#ifdef WIN32
+#ifdef _WIN32
 	HANDLE h = CreateFile(path.c_str(), GENERIC_WRITE, 0, nullptr,
 			      TRUNCATE_EXISTING, FILE_ATTRIBUTE_NORMAL,
 			      nullptr);
@@ -83,7 +83,7 @@ TruncateFile(Path path)
 void
 RemoveFile(Path path)
 {
-#ifdef WIN32
+#ifdef _WIN32
 	if (!DeleteFile(path.c_str()))
 		throw FormatLastError("Failed to delete %s", path.c_str());
 #else