De-duplicate release version

I'm fine with putting the release version in a header file, thus
necessitating the need to recompile every file that includes it if it's
changed, simply because it's not supposed to be changed that often.

The SDL_arraysize is necessary because sometimes we'll have subreleases
(e.g. 2.4.1, 2.4.2, 2.4.3), and who knows, maybe we'll get to 2.10
someday.

desktop_version/src/ReleaseVersion.h

@@ -0,0 +1,6 @@
+#ifndef RELEASEVERSION_H
+#define RELEASEVERSION_H
+
+#define RELEASE_VERSION "v2.4"
+
+#endif /* RELEASEVERSION_H */