Use constexpr constants instead of defines.

This is both to avoid using the preprocessor in C++ in general,
and to prepare for future usage of the library as a C++ module.

include/xparrow/xparrow_version.hpp

@@ -5,11 +5,11 @@
 *                                                                          *
 * The full license is in the file LICENSE, distributed with this software. *
 ***************************************************************************/
-#ifndef XPARROW_VERSION_HPP
-#define XPARROW_VERSION_HPP
+#pragma once
 
-#define XPARROW_VERSION_MAJOR 0
-#define XPARROW_VERSION_MINOR 0
-#define XPARROW_VERSION_PATCH 1
-
-#endif
+namespace xparrow
+{
+    constexpr int XPARROW_VERSION_MAJOR = 0;
+    constexpr int XPARROW_VERSION_MINOR = 0;
+    constexpr int XPARROW_VERSION_PATCH = 1;
+}