Add: support filtering content entries for patchpacks (#9541)

This changes nothing for us, but allows patchpacks to add the
right pieces of code to start filtering content entries on patchpack
only entries.

src/rev.cpp.in

@@ -72,6 +72,13 @@ const byte _openttd_revision_modified = ${REV_MODIFIED};
  */
 const byte _openttd_revision_tagged = ${REV_ISTAG};
 
+/**
+ * To check compatibility of BaNaNaS content, this version string is used.
+ * It should never contain things like "beta", but only the release version
+ * we are compatible with.
+ */
+const char _openttd_content_version[] = "${REV_MAJOR}.${REV_MINOR}";
+
 /**
  * The NewGRF revision of OTTD:
  * bits  meaning.