pgsql: Introduce frontend API able to retrieve the contents of PG_VERSI

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/cd0be131ba6f7e8b1323d188db20d15c2504b021

Modified Files
--------------
src/fe_utils/Makefile          |  3 +-
src/fe_utils/meson.build       |  1 +
src/fe_utils/version.c         | 86 ++++++++++++++++++++++++++++++++++++++++++
src/include/fe_utils/version.h | 23 +++++++++++
4 files changed, 112 insertions(+), 1 deletion(-)