Use PG_MAJOR explicitly if it is set
Use PG_MAJOR explicitly if it is set

On a recent Debian installation I ran into the issue that pg_config was
PostgreSQL 12, whereas only PostgreSQL 11 server binaries and libraries
were installed.

By passing on PG_MAJOR as --pg-version, we ensure that timescaledb-tune
will always use the correct version to make its decisions.

Currently, when building a Docker image for PostgreSQL 11 and installing
postgresql-common, timescaledb-tune fails as 12 is not (yet) supported
by timescaledb-tune.

timescale/timescaledb-docker