Due to [insert reason here], in the Debian package build process (and
only there), when building frontend code PG_VERSION_NUM appears to be
from the newest libpq-dev version installed, and does not necessarily
match the version of the server the code is being built against.
To work around this distribution-specific package build issue, we'll
define our own substitution variable which is taken from the value
provided in Makefile.global.
Note that the next release is intended to be 5.0 to coincide with the
release of PostgreSQL 12; 4.5 is currently a placeholder in case we
need to push out a feature release before then.
repmgr requires that the same "major version" (e.g. 4.3) is present
on all nodes, otherwise - particularly in the case of repmgrd - it's
highly likely things won't work as expected.
Implements part of GitHub #515.
