Codechange: Include flag for whether a build is of a tagged revision

src/rev.cpp.in

@@ -62,6 +62,13 @@ const char _openttd_revision_hash[] = "!!GITHASH!!";
  */
 const byte _openttd_revision_modified = !!MODIFIED!!;
 
+/**
+ * Indicate whether this is a tagged version.
+ * If this is non-0, then _openttd_revision is the name of the tag,
+ * and the version is likely a beta, release candidate, or real release.
+ */
+const byte _openttd_revision_tagged = !!ISTAG!!;
+
 /**
  * The NewGRF revision of OTTD:
  * bits  meaning.