Introduce frontend API able to retrieve the contents of PG_VERSION

get_pg_version() is able to return a version number, that can be used
for comparisons based on PG_VERSION_NUM.  A macro is added to convert
the result to a major version number, to work with PG_MAJORVERSION_NUM.

It is possible to pass to the routine an optional argument, where the
contents retrieved from PG_VERSION are saved.  This requirement matters
for some of the frontend code (one example: pg_upgrade wants that for
tablespace paths with a version number strictly older than v10).

This will be used by a set of follow-up patches, to be consumed in
various frontend tools that duplicate a logic similar to do what this
new routine does, like:
- pg_resetwal
- pg_combinebackup
- pg_createsubscriber
- pg_upgrade

This routine supports both the post-v10 version number and the older
flavor (aka 9.6), as required at least by pg_upgrade.

Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/aOiirvWJzwdVCXph@paquier.xyz

src/include/fe_utils/version.h

@@ -0,0 +1,23 @@
+/*-------------------------------------------------------------------------
+ *
+ * Routines to retrieve information of PG_VERSION
+ *
+ * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/fe_utils/version.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PG_VERSION_H
+#define PG_VERSION_H
+
+/*
+ * Retrieve the version major number, useful for major version checks
+ * based on PG_MAJORVERSION_NUM.
+ */
+#define GET_PG_MAJORVERSION_NUM(v)	((v) / 10000)
+
+extern uint32 get_pg_version(const char *datadir, char **version_str);
+
+#endif							/* PG_VERSION_H */